Soul marks. Not everyone got them; they only came if you were so inextricably connected to your partner that you'd never be able to be with anyone else, not even if they died. To a young Rose Tyler, the idea had seemed hopelessly romantic.

"How's it happen, Mum?" she'd asked as she traced the words on Jackie's arm.

Jackie had smiled bittersweetly and squeezed her hand. "If you have a soul mate love, you'll get your mark when you know you'd do anything to be with them."

Her mum had a second mark on her arm, the last words Pete Tyler had spoken to her. That's how it worked: first words when your soul became tied to theirs, last words when you were parted by death.

Rose spent her first three months with Jimmy examining herself eagerly in the mirror every day, hoping her soul mark had appeared. She spent the next six months praying every morning that it hadn't, although it would have been hard to spot an extra mark on her body. When she finally got out, she breathed a sigh of relief that she'd never been tied to him.

In all the time she dated Mickey, she never once looked for a mark. Mickey was comfortable, like a warm jumper, but he wasn't the other half of her soul. Knowing that made it easy to run into the TARDIS when the Doctor asked the second time.

When his idea of a first date was to take her to watch her planet burn, she wondered why he'd asked her to come in the first place. Then he told her about his home, and something tugged at her heart. She needed to do something to ease the bleak loneliness on his face, so she slipped her hand through his and said simply, "There's me."

When they were trapped in a basement in Cardiff surrounded by ghosts, she saw the apology in his eyes when he explained the seeming paradox of dying before she was even born. She wanted to reassure him that she didn't blame him, so she shook her head and reminded him that she'd wanted to come.

When he took her home twelve months late and they ended up locked in 10 Downing Street, she saw the desperation in his eyes when he uttered the words, "I could save the world, but lose you." She knew then that even though he was this amazing genius, he needed her strength, and she gave it to him, refusing to accept that she would die in that room. And they hadn't.

Each moment, each crisis had imprinted the Doctor more firmly into her heart, but it wasn't until he threw away his weapon after facing his oldest enemy and clung to her desperately that she knew she loved him.

When she found a dark spot on her hip the next morning, she figured it was a bruise from all the running and dodging she'd done in Van Statten's bunker. It wasn't tender, which was a bit weird, but what else could a small, dark spot on her skin be?

The spot ached when the Doctor called Adam her boyfriend. The word was just wrong, even though pre-Doctor Rose would definitely have been interested in Adam. Riding the elevator up to Floor 500 with the Doctor, standing with him even when he said she could stay below, that felt right, and the ache dissipated.

She forgot about the mark for a while, but then came the disaster in 1987. The moment the Doctor was swallowed by the Reapers, Rose felt a searing pain in her hip. She wouldn't lose the Doctor, not because she'd done something so stupid. Watching her dad die hurt, but it didn't hurt as much as the thought of life without the Doctor.

When she looked at her hip that night, she wasn't surprised to find the undefined mark was now a single word, etched clearly into her skin. Run.

After the Doctor sent her home from the Game Station, the first thing Rose did was look for her second mark. She stared at her hip for ten minutes, and when the words, "Have a fantastic life," didn't appear, she knew that somehow, she would get back to the Doctor.

When Rose saw the Doctor explode into golden light, she thought for sure that was it. The new man, with his bouncy personality and shock of brown hair, claimed he was still the Doctor, but she refused to believe him. She knew she'd see her second mark when she changed for bed, and she hated this… this imposter for not leaving her alone to grieve.

But then he did something she never could have expected. His entire demeanour shifted from exuberant to intense, and he looked down into her eyes. "Then how could I remember this? Very first word I ever said to you, trapped in that cellar, surrounded by show window dummies… oh… such a long time ago. I took your hand…" He took her hand, and it fit hers the way the Doctor's always had. "I said one word. Just one word. 'Run.'"

The mark on her hip burned when he said it, and her heart was his from that moment. Oh, sure—it took her mind a bit to catch up, but really, with such a fantastical chain of events, that could only be expected.

No matter what he looked like, they were the Doctor and Rose Tyler, and they were going to have a fantastic life.