Disclaimer: One Piece? Not mine. The idea of Devil Fruits? Not mine. The Gaki Gaki no Mi? Who wants it?
Characters/Pairings: Straw Hat Pirates. Zoro and Sanji bonding, with hints of potentially-maybe-in-the-far-future slash.
Timeline: Stuck in the anime timeline (because it's the only way it works) between the Davy Back Fight and the meeting with Aokiji.
Warning: We have grown up (more or less) people turning into children here. If you don't run away now, please don't act like you haven't been warned.

Prologue

"I have good news," Usopp declared as he slammed open the door of the galley. "But I also have very bad news."

Sanji, who had been staring at the two unconscious forms huddled on the table under a light blanket for the past hour, looked at him blearily. "Shoot."

"They've been turned back to six-year-olds."

Sanji rolled his eyes. Nami, who'd come back with Usopp but apparently didn't know any more than Sanji did, smacked Usopp up the head. "Thanks for pointing that out," she snarled. "Anything we don't know?"

For a moment the sharpshooter looked about to make a whiny comment about how dare she hit the Great Captain Usopp, then he remembered that he had actual information to give.

"Well, it's like that..."

The townspeople had been a lot of help. When he'd got there to explain that he'd found two of his crewmates unconscious and ten years younger than usual, Usopp had expected to get laughed at. Instead, he was the witness to a sharing of meaningful looks and a collective sigh.

The culprit, it turned out, was called Fletan. He was a merman of the halibut variety, arrived there about twenty-three years earlier after having been rejected by his kin for being too nicely-spirited towards humans. The local fishermen, after a few amusing encounters that sounded a lot like Chopper's first meeting with the Straw Hat crew, had finally seen how much use he could be and adopted him as one of them. Fletan, an extremely good-natured and helpful individual, was soon fully integrated in the population, and it seemed that he'd finally found his place in life.

Until, ten years after his arrival, a dreadfully familiar flag was spotted on the horizon. They were raiders, he remembered, detached from the main group and basically free to do whatever they wanted. Which was, he remembered, along the lines of pillage and plunder. He remembered his unhappy childhood, remembered being endlessly abused by those people he'd thought of as brothers, and decided that now that he had found the place and people that had unconditionally accepted him, he would be brave and stand up to the invaders, ready to give up his life if it was what it took to--

"Can you please cut down on the storytelling and give us the facts?" Nami interrupted, looking mildly annoyed. Usopp pouted.

"Well, Fletan knew he would never be able to take them all down by himself, so he made a terrible bargain," he said, and paused for dramatic effect. "He ate the Devil Fruit that had been the village's treasure for generations."

"A decades old fruit? Can't have tasted that good," Sanji commented, opening his second pack of cigarettes since they'd docked on this wretched island.

"Devil Fruits always taste bad no matter how fresh they are, cook-san," Robin provided, smiling at him. Sanji beamed, and was able to offer to prepare her a fruit salad to wash off the residual taste when Nami coughed meaningfully.

"Back to topic, if you don't mind? So he ate a Devil Fruit."

"Yes," said Usopp. "It turned out to be the Gaki Gaki Fruit."

"The Ga... is that even possible?" Nami asked doubtfully as Chopper stared in amazement. Usopp gave a long-suffering nod. "It's a beam, like the Foxy guy? Anyone the beam touches gets turned into a six-year-old."

"Who the hell makes these things anyway?" Sanji asked, rolling his eyes. That was the last thing he needed, having to deal with two brats that were troublesome enough as almost-adults. "At least tell me there's a way to turn them back."

Usopp sulked at him. "I'm not there yet!" he protested, then went back to storytelling mode.

"Of course, the Gaki Gaki Beam was not enough to take care of all the mermen pirates, even though he managed to catch some of them unaware. Despite his and the population's valiant efforts, they soon took possession of the town, and through murder and abuse and tax collection-"

"Was their captain possibly called Arlong?" asked a white-faced Nami. Usopp blinked at her in realization. "I... didn't ask," he admitted. "Could have been."

"So they murdered some people, stole all the money, and so on. Spare me the details and move on," she ordered. She looked like she was about to be sick, so Usopp gave up on the tale of terror he'd carefully crafted while coming back from his information hunt and complied.

"They beat Fletan up, but didn't kill him. By the time he was allowed out of the doctor's care, they were gone, and he decided to make things go back to what they used to be. But when he tried to help the fishermen in their work..." Usopp trailed off.

"He found he couldn't swim anymore," Chopper completed, seemingly on the verge of tears.

"He found he couldn't swim anymore," Usopp said dramatically, glaring at Chopper for stealing his line. "It drove him insane."

"Why?" Sanji asked. "I understand he ate the fruit for a bad reason, but he wouldn't be the first one who can't swim..."

"But a merman unable to swim? It must have been hard on him," Robin contemplated. Nami nodded in approval.

"Probably. They never feel good if they can't go swimming at least twice a day. It's no wonder that the guy went mad."

Feeling that he was losing his public's attention, Usopp went back to the story. "Now he lives further inside the grounds. Spends most of his time sleeping, but sometimes he goes back to the coast and attacks everyone he sees."

"And no one has gone to stop him?" Sanji asked in disbelief. It couldn't be fun, finding your husband or your mother reverted to a kid. He couldn't believe no one had taken the matter in their own hands.

"But, but, it's not his fault," Chopper said, tears shining brightly in his eyes. "That's so unfair," he sniffed. Usopp nodded.

"The villagers remember him and what he did to help them, so they leave him in peace. It doesn't happen that often, and the effect wears off after thirty days. So everyone bought children's clothes, and they just wait it off."

Thirty days. Thirty days. Thirty days. Thirty days. Sanji could see them already, playing and wreaking havoc in his kitchen for a month. This was not going to end well, he could tell. But he'd faced worse things. And Nami-swan and Robin-chwan would be so impressed once he proved to them that he could be the perfect father. "Alright," he said, trying to see the good side of things. "What's the good news?"

Usopp looked somewhat embarrassed.

"Uhhh..." said Chopper, like he wanted to say something that he wasn't sure would be well received.

"I rather think that was the good news, cook-san," Robin-chan said, smiling sweetly at Sanji. It was almost enough to make him forget that things could actually get worse. 'Almost' was such a terrible word.

"There's a fleet of Marines headed our way," Usopp said, now looking excessively uncomfortable. "They'll be here in four or five hours, and from what the grocer said, their captain is always willing to go a bit out of his way to get his hands on a bounty."

"Oh, good," Nami groaned. "And at the moment, two of our bounty heads are absolutely unable to defend themselves."

"I can take care of Marines, Nami-swan," Sanji offered, lighting a cigarette and feeling very cool as he did so. But contrary to what he'd expected, no one in the room seemed really enthusiastic about his suggestion.

"You and whose army?" Usopp asked after a few seconds of tense silence. Sanji turned towards him and glared. At the edge of his vision he saw a Rumble Ball halfway to Chopper's mouth.

"What is that supposed to mean, long-nose?" Sanji sneered. Usopp looked mildly scared, but held his ground.

"It's a FLEET! Merry will never survive a direct confrontation!"

"I can end it before it gets to that!"

"Yeah? One of their snipers gets a lucky shot, we're all going down!" Both of them ignored the sudden burst of fur at their side.

"And what will you do to st-"

"ENOUGH!"

The schoolteacher harmonics in Nami's voice and the loudness of her fist crashing on the table made them both shut up mid-yell. "This is not the time for fighting among ourselves," she said, glaring around and daring them to contradict her. "The log is set. We can leave now. But that choice lays with the captain, not any of us."

They all looked at the children, whose sleep apparently hadn't been disturbed in the slightest. "The captain... is unable to make the decision", said Chopper, gone back to his usual shape but hiding behind Robin in usual Chopper fashion.

"In that case, the first mate is in charge," Robin offered helpfully, her chin resting in her own hands and a third one stirring her cup of coffee. They all looked back to the table. Drool was dripping from the corner of the mouth of the miniature Zoro. No help would be coming from there.

"And if the first mate can't do it?" Nami asked in a dangerously strained voice.

"Chaos," Robin answered with her usual bright smile. There was a collective sigh. For a moment, Nami seemed to consider quitting the crew there and then. Then there was a shuffle, and all the adults in the room froze as Luffy pushed himself to a sitting position.

He blinked sleepily and looked around the room, slowly, like he was trying to remember if he was supposed to know where he was. Having decided he wasn't, he glanced at each of them in turn, apparently unaware that they were all staring back expectantly. The sight of Chopper seemed to puzzle him for a moment, then a huge grin broke on his face as he reached a conclusion.

"Food?" he said hopefully. Nami heaved a loud, long-suffering sigh.

"Everyone who feels that Luffy isn't any less qualified than usual to make a life-or-death decision, raise your hand," Usopp muttered.

More hands were raised than a crew of seven should reasonably possess.