Disclaimer: I do not own any part of Sherlock; it all belongs to the BBC, Mark Gatiss, Steven Moffat, et al. I write these stories purely for enjoyment; no copyright infringement is intended.

Author's Note: There are some complicated notes that go with this story, most particularly with the title, which I have put at the bottom. As always, my deepest thanks to WickedForGood13 and Nagaem_C, who both encouraged this little project with much enthusiasm and discerning reading.

Triquetra: shape formed of three vesicae piscis, sometimes with an added circle in or around it. Also known as a "trinity knot," the design is used as a religious symbol by both Christians and polytheists. In romantic symbolism, it is frequently used to represent the promises of a relationship, such as to love, honor, and protect. It can be used solely as a symbol of protection. It is similar to, and probably related to, the valknut, which is a shape formed of three interlocking triangles that was used primarily in Germanic paganism. The valknut was associated with Odin, the Old Norse God of war, battle, victory, death, wisdom, Shamanism, poetry, prophecy, magic, and the hunt. Given this lineage, it seemed utterly appropriate to use "Triquetra" as the title for a story about this formidable OT3, with all of their complex facets. (All of this information is cobbled together from various Internet sources, so my apologies for any inaccuracies.)

It should be clear from this first chapter that, for the purposes of this story, I'm 1) going AU in the middle of "The Sign of Three," with the intent of thinking about what would have happened if Mary had gone to Sherlock earlier, 2) assuming that the lovely Mary we see in TEH and TSoT is the "real" Mary, or at least real enough that her actions in HLV were out of self-preservation more than anything, and 3) that I am playing with the wonderful idea of Johnlockary.

Triquetra

Mary stood on the pavement in front of 221B, contemplating the conversation she was about to have and the small hints that had brought her to Sherlock's door. She had been thinking for several weeks, ever since Sherlock had made his dramatic reappearance at her truncated engagement dinner, and her thinking had become even more serious once she had sussed out how terrified Sherlock truly was about her and John's upcoming wedding.

She had started paying closer attention to the things Sherlock did and didn't say, and it had become clear to her that Sherlock was very deeply in love with her fiancé, even if said fiancé was too oblivious to see it. The only thing holding Sherlock together was the need to make John happy, even if that happiness had nothing to do with him. Oh, John was ecstatic that Sherlock was back – in fact, she'd never seen him so cheerful and content, once he got over his anger at the detective's deception – but his priority was her, and them, and it was eating Sherlock alive in a silent, bone-deep way that Mary wasn't sure she could watch any longer. It was heartbreaking, and there was no need for it to be so. Not if she could bring them both around.

She took a deep breath and knocked at the door of 221B, summoning up a friendly smile as Mrs. Hudson answered the door. She loved Sherlock's landlady, but at the moment she was too worried and nervous to find a completely sincere smile.

"Oh, hello dear," Mrs. Hudson said as she opened the door. "Were you looking for John? He's not here as far as I know."

"Hello, Mrs. Hudson. No, actually, I was looking for Sherlock," Mary answered. "Is he in?"

"I think so, dear; I haven't heard him rush out anywhere, and he was playing the violin until about half an hour ago. Go right on up," Mrs. Hudson answered cheerfully.

"Thank you," Mary said. She went up the stairs purposefully, not hurried and not slowly, for she was sure Sherlock was listening and would deduce that she had come deliberately.

"Hello, Mary," Sherlock said calmly as she walked into the flat. He was standing at the window, hands shoved in his trouser pockets, wearing his crimson dressing gown over his Oxford shirt and suit pants. His tone wasn't cold, exactly, just contemplative, as if he had been in the middle of several trains of thought – and of course, he probably had been, Mary reminded herself.

"Sherlock," she said warmly, and the smile that broke over her face this time was genuine. She really was attached to this odd, brilliant man – for as short a time as she had known him, he had become tremendously important to her.

Sherlock turned from staring out the window to look at her, and he gave a thoughtful frown as his eyes scanned her. "You came to see me today because John took a weekend shift at the surgery and you knew I would be here. You went to look at cakes this morning, which you told John you were going to do, and then caught a cab to come to Baker Street. You didn't want us to be interrupted, and you knew that John would be busy with patients after lunch."

"Right as always," Mary said, her tone both sheepish and admiring, for it was impossible not to be awed by Sherlock's intellect. "I came here for two things, actually, and I need your help with both of them, Sherlock."

"Not always," Sherlock murmured, his eyes narrowing so slightly, but he gestured toward the two chairs in front of the fireplace, and they moved over, Mary settling herself in John's chair while Sherlock sat in his accustomed leather chair. Mary saw the flash of pain in his face as they sat, before he could hide it, seeing her where John usually sat. It only strengthened her resolve.

God, people could be ridiculous. And these two men, these two impossible idiots with whom she had bound up her life, were quite possibly the most ridiculous pair she had ever seen.

She leaned over, propping her elbows on her knees, and reached for one of Sherlock's hands, clasping it between her own. He stilled in surprise, but let her, waiting to hear what she would say before he reacted. They had achieved their own level of trust since they met, and while Mary was not foolish enough to believe that Sherlock trusted her entirely, not quite yet, she knew that he trusted her because John did. For now, that would have to be enough, and she could only hope that this conversation didn't shatter it.

"First, we need to talk about the fact that you're in love with John," she said, and she felt Sherlock's hand jump in her grasp. She wrapped both her own hands around his fine-boned, slender digits in reassurance.

"Mary . . ." Sherlock began tentatively, and she looked up.

"Don't deny it, Sherlock," she said, gently but firmly. She hoped that her face conveyed the same reassurance that her hands had. "Please don't. Don't dishonor your beautiful heart or my intelligence by denying it."

Sherlock stood up in one smooth swirl of limbs, muscle, and dressing gown, pulling his hand free as he did so. Almost before Mary could blink, he was back at the window as he had been when she entered, facing away from her, voice carefully neutral.

"It's not relevant," he said evenly, though she could hear the fine thread of a tremor in the words. "It doesn't matter."

"Not relevant?" Mary exclaimed, standing in her turn. "Sherlock, of course it matters."

"No, in the grand scheme of things it really doesn't," Sherlock said, and there was just a hint of the old arrogance and superiority there, the "Let me tell you exactly why you're wrong" tone that Sherlock was so famous for, but there was also self-deprecation, and exhaustion, and resigned acceptance.

It made Mary want to weep.

"John is alive, and happy, and he has found a woman who is, astonishingly, everything he needs," Sherlock continued, flashing Mary one of his rare real smiles, even though it was shot through with sorrow. "That is all that really matters, all that is really of any importance. And perhaps . . . it is better."

Mary stepped up to him, up to where his face was so guardedly turned away from her, and he continued to stare out the window as she stared at him, willing him to give her some answers. "Better? Better than what? Sherlock, you can't possibly think he is better off without you."

"Isn't he?" Sherlock returned. "I hurt him, Mary. I . . . miscalculated, badly, and hurt him more than I ever would have wished to, and he would not have survived that if not for you. So yes, better all around, don't you think?"

He turned and made to move past her, but Mary blocked him deliberately. She was going to get him to understand what she wanted, and even more importantly, to see that he was so very wrong about John, but she wasn't getting anywhere with gentleness.

Time for some tough love, then.

"Sherlock, why did you jump off that roof?" she asked bluntly.

Sherlock took a tiny, involuntary step backward, and his eyes widened in confusion. "I told you why."

"You told us the obvious why – that Moriarty and his network needed to be finished, and believe me I am grateful that you took that task upon yourself," Mary replied. "No one else could have done what you did. But you didn't tell us the true reason why, Sherlock. There had to be other ways to stop Moriarty than faking your own death. If it had just been a question of you versus him, I can't imagine you wouldn't have found some other way to best him and deal with his associates. So what was he holding over you? He had to have something; he even shot himself before you jumped, and you jumped anyway. What leverage did he have?"

Sherlock was still staring at her, wrong-footed and speechless at the way she'd turned the conversation, and Mary pressed her advantage while she had it. "The way I see it, the only thing he could have held over you that would have mattered that much was John's life. You jumped, you faked your death, you went on a two-year solo mission that almost got you killed countless times, because it meant that John would live. Moriarty had set it up, somehow, so that John was still under threat even after Moriarty was dead."

"Yes," Sherlock whispered, so quietly that Mary wouldn't have heard it if she hadn't seen his lips move. His eyes were stricken, although Mary didn't know if it was from the memory of that day or from her rapid uncovering of his secret.

Sherlock cleared his throat and moved again, and this time Mary let him, watching him walk over to the mantel and fiddle restlessly with the skull. "There were snipers, Mary. Not just on John, but on Mrs. Hudson and Lestrade as well. I would not let harm come to them, either – but just the threat on John would have been enough." His voice had dropped back down to a whisper by the end, and he ran a hand through his hair, once again keeping his face averted.

"And you don't think it would mean something to John, to know that?" Mary said incredulously. "Sherlock, you saved his life! You put yourself through two years of hell for him! You don't think he would want to know? At the very least it would explain to him why you didn't tell him."

"I do not need his gratitude," Sherlock spat, finally turning around and looking at her, and Mary recognized this expression from John's stories, the icy eyes, the hard tone that shredded people's minds and hearts to ribbons. She had not seen it directed at herself before, but she recognized it, and it was lucky for her that she was not someone who was easily cowed. "I did it because I love him, because I can no longer conceive of a world where he does not exist, but I will not have him tied to me out of obligation or pity. I will not allow those things to be part of our friendship, Mary. I could not bear it. I saved him because there was nothing I wanted more than for him to live and be happy. I could not assume I would live, and even had I known I would make it this far, I would never want him in my life out of some misguided sense of duty. I have his friendship and his forgiveness, and it is enough."

"It is not enough," Mary contradicted sharply. "Not for him or for you. He is already tied to you, Sherlock – was already tied to you, before you jumped, and not out of obligation or pity or any of those absurd things, but tied to you through love. I know you didn't see him while you were gone, and I did, but Sherlock, can you really believe that he would have been so angry with you when you came back if he had not loved you so much? He was a shell when I met him, living less than half a life, because the man who was his entire world was dead, and John had never so much as tried to say how he felt, hadn't even realized it until just before the end – what he thought was the end," Mary amended. Her breathing was rapid and her chest tight, and the need to shake the man in front of her was almost stronger than she could stand.

"John isn't – he didn't –" Sherlock fumbled, unable to articulate what he wanted to say through the flood of new information. He was looking at Mary as if he'd never seen her before, and maybe he hadn't, though Mary was willing to bet that what she had just said was reframing every image of John he had ever captured in his formidable mind.

"Not gay; yes, I know," Mary said, permitting herself an impish smile. "Are you?"

Some of the tension left Sherlock's shoulders and posture as he looked at her expression. "Not strictly speaking, no," he murmured. "I don't find most terms surrounding sexuality terribly helpful. Pansexual or demisexual, maybe. Perhaps both. Perhaps neither."

"But you love John," Mary said, taking a step toward him.

"Yes," Sherlock sighed, and the exhaustion was back, permeating every line of his frame. Mary ached to see it; she wanted so badly to make it go away, but she wasn't the one who could. Not the primary one, anyway. She could only get all of them on the right path.

"And you want him." Another step.

Sherlock's hands curled helplessly into fists. "Yes. What are you saying, Mary?"

She took the third step, so that she was close enough to rest her hands on Sherlock's forearms. "I'm saying that I don't think it has to be either/or, Sherlock. I'm saying that I'm tired of seeing my fiancé and his best friend be utterly miserable because they are without each other, when they've been longing for each other since the day they met. I'm saying that my heart hurts because I love John, and I care about you so very much, you utter madman, and I think there's a way to heal all of these hurts and empty places between all of us. As long as we're all willing to do this, I think we can make it work."

Sherlock was looking at her with something akin to open wonder in his eyes. It was difficult to impress Sherlock Holmes, and it made Mary warm in every way imaginable to know that she had done so.

"You . . . are a marvel, Mary Morstan," Sherlock said softly.

She smiled fondly at him, her customary sauciness slipping back into her tone. "Why do you suppose John loves me? I put very little stock in rules and conventions, Sherlock, much like someone else I know."

"You don't have to do this, Mary," Sherlock reminded her quietly. "I would not trade my happiness for your pain, any more than I would John's. You make him happy, and he makes you happy, and I can be content with that. What you are proposing is . . . unconventional, and you might find it hurts you more than you think it will."

"I know that I don't have to," Mary answered him, squeezing his wrist lightly. "I want to. I've been thinking about this for weeks, Sherlock, watching you and John, listening to my own heart, and I'm not proposing this idly. We are the two halves of John's heart, and I would not ask him to live another half-life. I want more than anything for him to be whole."

Sherlock shook his head slightly. "Even if you are right about . . . how John feels, I can't imagine he will agree."

"I am, and he does, and he will," Mary said with a grin, back on sure footing. "You leave that to me."

Sherlock assessed her again, his eyes filled this time with the vulnerable warmth Mary had seen in him from that very first evening, the tentative reaching out of a heart that was accustomed to far too much hurt. She could see him choosing his words very carefully when he spoke again.

"And you . . . tolerate me . . . enough to do this? To offer this?"

And if Mary thought her heart had hurt before, just hearing that question from Sherlock's lips made it break a little more.

"Were you not listening earlier?" she chided him softly, sliding her hands up so they rested just inside his shoulders, over his collarbone on both sides. "I do much more than tolerate you, Sherlock. I'm very fond of you. I would never want anything to happen to you, not just because it would destroy John, but because it would hurt me as well. You are so much more than the face you show to the world, and I find I'm beginning to care very much for the man underneath."

Sherlock went completely motionless then, and Mary could see him struggling to process what she'd said, struggling to wrap his mind around the idea that she could care for him on his own merits, and not just as John's best friend.

"It doesn't have to be everything all at once, you know," she reminded him, giving him another smile. "We can see where it goes, where the boundaries are, physical and otherwise. The only thing I need to know is, do you care for me enough, in some way, to try and do this?" she asked, reaching up and brushing a fond hand over his cheek.

Sherlock was back to studying her now, that bright, intense gaze taking in as much as it could, stripping her bare, and oh, it was intoxicating. No wonder John couldn't get enough of this man.

Sherlock caught her hand in his, where it rested on his shoulder, and let his eyes slide closed. "Yes." Just a breath, barely there. "Yes."

"Good then," Mary said, leaning in to brush her lips over his knuckles. His eyes opened in surprise, but she had already stepped back, simply letting the little gesture stand as its own sign of affection.

"That brings us to the second thing," she continued, and now her hands came up to wring themselves together. "It's not nearly so pleasant, I'm afraid."

Sherlock's eyes had become analytical again, but just a hint of a sly smile hovered at the corner of his mouth. "Are you finally going to tell me what you've been lying about?"

"I – yes," Mary stammered, surprised. "How much do you know?"

"Not much. I wasn't trying. Luckily for you, I knew that you weren't lying about anything to do with John," Sherlock said gravely, and the look in his eyes was enough to make Mary shiver. He would have had no compunction about destroying her if he had caught any hint that she wasn't sincere in everything she felt and did for John. He had done much worse in the two years he had been away, she knew – and for just a moment, every ruthless minute of those two years showed in his eyes.

"Whatever you were lying about, I put it aside because John trusted you, and because I could see that you loved him," Sherlock went on, a trifle more kindly. "But something's changed. Why do you need me?"

Mary took a deep breath before pulling a chain out from under her blouse, fingering the thumb drive that dangled at the end of it. "Someone is after me. Or rather, they're after the person I used to be."

Sherlock gestured back toward the chairs, everything in his posture reverting to the alert detective, intent on a case. "Tell me everything."

Sherlock watched her for a long minute when she finished, two hours and several cups of tea later. His fingers were steepled at his lips – had been for ages, Mary realized – and his expression was a curious mix of intrigued, sympathetic, and angry.

"You have to tell him," Sherlock said flatly, the first words he'd spoken in well over half an hour. It was a statement, not a question, and Mary flinched.

"I know," she said softly. "I should have told him before now, but I thought - I thought we'd have time, and then suddenly Magnussen was breathing down my neck, and I didn't know what to do."

The anger won out in Sherlock's face, and Mary gripped the edge of her seat, knowing whatever came next was going to be hurtful.

"And how exactly would telling John after your wedding make anything better?" Sherlock said harshly, unfolding his long frame from the chair so that he could pace through his agitation. "'Oh, by the way, darling, I'm sorry I didn't tell you before we got married, but I used to be a CIA assassin and then a freelancer. I've given that up now, though, found a false identity and everything, no need to worry.'"

Mary felt a flush of anger and shame suffuse her cheeks, and she spoke before her brain could catch up with her mouth. "I hardly think you're one to talk about withholding information from John," she snapped.

Sherlock stopped, fury in every line of his body, and when he turned to face her, the look in his eyes could have cut glass. "No, I'm not, but I did it to save his life, Mary. I was willing to lose him and I knew I might. Are you prepared to face that possibility? I will not let anyone harm you; I will find a way to deal with this, but I will not be a party to violating John's trust yet again. You have to tell him and let him decide what that means for the two of you. Nothing else happens until he makes that decision."

"Yes, of course," Mary agreed falteringly. She understood Sherlock's logic, and really, it was the advice she would have given anyone else. She was used to keeping secrets, but there were limits. She could not spend her whole life with John and not have him know what she had been before she had become Mary Morstan. Had she not been so afraid of losing John, how he would react to one more of his loved ones having potentially deadly secrets, she would have told him long ago. She had suspected that coming to Sherlock for help would result in exactly this ultimatum. If she did not tell John, Sherlock would, and it would break John's faith in her irrevocably.

Of course, the secrets she had to tell might do that anyway. Sherlock was right; no matter how much she loved John and wished to be his wife, she had to accept that John might not want to continue their relationship, once he knew what she had been. That had to be his choice. Perhaps she had, subconsciously, come to Sherlock not only because she wanted things to work between the three of them, and because she needed his help with Magnussen, but also because she needed his external force to make her face her own fear, to help her do the right thing.

As Mary thought, she registered Sherlock talking quietly to himself, thinking aloud and trying to assimilate all of the information she had told him. The meaning of what he was saying slowly filtered back into her brain, as her mind came back to the living room of 221B.

". . . It doesn't make sense, it doesn't make sense, why would Magnussen want you?" Sherlock questioned, back to pacing in front of the fireplace. "You have a particular skill set, but why would it matter if you were ex-CIA? If he wanted to use your skills he would keep your secret, although it's possible he could try to blackmail you into doing his bidding. The only thing accomplished by exposing you is potentially ruining your relationship with John; you have no relationship to anyone important on this side of the ocean . . . oh."

If Mary had thought Sherlock was angry before, it was nothing to the look on his face now; he looked absolutely feral as he whirled around and gripped her wrist. She shrank back, actively willing her body not to resist; starting a physical fight would only confirm her guilt in his eyes, whatever he thought she was guilty of. Trying to fight him wouldn't have done any good, anyway; the Sherlock who was holding onto her tightly enough to bruise and potentially with enough leverage to break her wrist was absolutely terrifying in his ferocity.

"Are you working for Magnussen?" he demanded, his face inches from hers. "Is that what this was, Mary? Is that what our little conversation before was all about?"

"No, Sherlock, what are you talking about?" Mary exclaimed. "Why on earth would I be working for Magnussen, when he's the one blackmailing me?"

"Oh, I think it's fairly simple," Sherlock snarled. "Magnussen believes in pressure points. You on your own are not that valuable, but he knows that you are engaged to John, he has to. Maybe that's what the bonfire was about when I first came back; he was warning both of us. You want the information he has on you to go away, so he offers you a trade. Take your story to Sherlock Holmes, your fiancé's best friend. Gain his sympathy, get him to help you, befriend him, offer him everything he ever wanted emotionally. Find out everything you can about his habits, his family, what his weaknesses are. You give Magnussen as much information as possible on me, and he leaves you and John alone for the rest of your lives. He can then come after me whenever and however he pleases. If you don't do what he wants, he's already made it perfectly clear he can kill your fiancé whenever he likes and expose you to the world as a former killer. Your pressure points are your former life and John; mine is John and Magnussen knows that, so he uses John against both of us to get to me. I am out of the way eventually, if not immediately, and you get your perfect little life with John."

It made a frightening amount of sense, and there was no way to prove to Sherlock that he was wrong. Mary looked him straight in the eyes.

"Sherlock, I am not working for Charles Magnussen. I swear it," she said steadily. "I have nothing to give you but my word, but it is the truth. I came to you because I have no idea how to find my way out of this, but I would never, never work for that reprehensible man. Even I have some sense of morality. In fact, in my other life I would have gladly accepted the contract that would have rid the world of him. People like Magnussen need to be put down; that's why there are people like me – like I used to be."

She reached down with her free hand, since Sherlock still hadn't let go of her, and clasped the thumb drive that was still around her neck, pulling its chain up and over her head. She held it up, still looking at Sherlock unflinchingly.

"Take it," she said. "My whole other life is on here. Who I was, how I came to be CIA, every job. You can read it if you like, keep it for insurance, destroy it if you are willing to be that forgiving. I don't know if Magnussen has an exact copy, or if he only has some of it, but he has enough. I have no idea how he got it. As far as I know, no one else has that information, and I am willing to trust you with it."

Sherlock reached out and curled his fingers around the drive, letting her wrist go at the same time. He looked at the drive in his palm and raised an inquiring eyebrow.

"A. G. R. A.?" he questioned.

"My initials," Mary filled in quietly.

Sherlock studied her for several seconds and then, to her utter and complete surprise, he leaned down and gently kissed her forehead. Mary inhaled shakily; the absolution in that brief press of lips left her light-headed.

"I am sorry," he murmured. "I had to be sure."

Mary shook her head. "You can't be sure," she said helplessly. "I have no proof."

"Your body language is proof enough; guilty people do not display the same reactions," Sherlock said with certainty. "You have tremendous control over your physical tells, Mary, but even you are not infallible."

"Thank you, I think," Mary said with a wry smile. Her face became serious again as she looked up at Sherlock. "Surely it can't have escaped you that Magnussen can achieve the same ends even without my cooperation," she pointed out. "He had to know I would go to you with the skip code; he was warning us both. He has the information about my former life, and the means to get to John, who is my other pressure point. John is your pressure point. Therefore, by controlling me he controls John, and by controlling John he controls you. The result is the same."

"Yes," Sherlock said thoughtfully. "The thought had occurred. What I don't understand is why; why would Magnussen want me? I cannot give him anything he doesn't have already."

"What will you do?" Mary questioned. Now that the decision was made, she felt an overwhelming sense of relief. She was still terrified of telling John, but simply knowing that she must made it easier. Leaving the matter of Magnussen in Sherlock's hands was tremendously reassuring.

"I cannot approach Magnussen the way I would a normal case," Sherlock answered, his fingers steepled again as he considered. "He is far too powerful, and he does not live in the shadows in the same way Moriarty did. We don't want him to use his leverage, so we must find a way to go after him that keeps him from using it. I may have to bring in Mycroft, Mary. Much as I hate to ask him for anything, he does have his uses, and in this case he may be our only recourse."

"Do what you have to, Sherlock," Mary said resignedly, standing up and pulling her coat on. "I trust your brother's discretion almost as much as I trust yours; for all I know, he may already know who I was before."

"It's possible," Sherlock acknowledged. "I will talk to him."

"Thank you," Mary said, and before Sherlock could object, she folded him into a hug. "Thank you for trusting me. Thank you for caring."

She felt Sherlock nod against her hair. "Tell John, Mary. He is the wisest and best man I have ever known; he will understand."

"I hope you're right," she whispered, and giving him another squeeze and a quick smile, she made her way out of the flat.

As she stood on the curb and raised her arm for a taxi, she let out a long sigh. Now she just had to figure out what on earth she was going to say to her fiancé.