They dropped into the sewer and splashed into the darkness. Raph's mind would not stay still. The strange Donatello loped easily through the tunnels ahead of them. It can't be. But it was. Donnie had been lying in the glass, and now he was...what?

"So what happened, Donnie?" He stumbled over the name. It didn't fit the self-assured warrior who strode ahead of them.

Mikey stopped dead. "That's not Donnie."

"Yes, it is," said the turtle.

"Buuut-" said Mikey, "you're old."

"I grew up." There was dry amusement in the tone.

"What, in 10 seconds?" snapped Raphael. He shoved Mikey forward. "You might look like Donnie, but you're not-"

"Are you really going to start a fight now, Raphael? Wasn't this morning's bust-up enough for you?" The cold, dead voice echoed against the brickwork.

Raph's breath caught. He remembered nearly coming to blows this morning with Donnie when another one of Donnie's experiments had caught fire. "How did you know that?"

"Because I was there." He came to a crossways and stopped. "Which way?"

Raph drew his sai in one smooth motion. "If you were Donnie you would know the way to the lair. You-"

The turtle made an exasperated sound. "I haven't been in these tunnels for fifteen years, Raphael."

"But-"

The turtle turned around. "Look at me. Who do you think I am? Stop being a child and tell me which way to go."

Mikey's head was turning from one to the other in confusion. Raphael shoved him ahead. "Lead the way, Mikey."

Donatello turned and followed Mikey without a word. Raphael's fists clenched. He couldn't deal with this. Couldn't deal with Leo's slack face, with the mangled arm. He needed something to hit. That much he understood. His eyes focussed on the strange Donatello's back, and he gripped his sai tighter. I'll start with you. As soon as Leo is patched up.

They made their way through the tunnels in silence, Raphael right behind him, ready for treachery. So when they pushed through the turnstiles and into the lair, he saw Donatello's shoulders jerk, saw the catch of breath. For some reason the lair startled him. Why?

Mikey ran for the dojo, shouting for Splinter. Sensei appeared at the top of the steps and froze, taking in Leo being carried by the strange turtle. He scanned the group as he hurried down the steps. "Where is Donatello?"

"I'm here, Sensei." The voice was even more dead than before, and the muscles on Donatello's shoulders were knotted with tension.

Splinters eyes widened. Right there with you, Sensei, Raph thought.

Donatello gave Splinter an awkward, rusty bow, as if he hadn't made the gesture in a long time.

"Donatello?" There was confusion and disbelief in Splinter's tone.

"Yes." The voice was strained. Donatello cleared his throat. "Leo is hurt."

That was enough to still the questions Raph could see rising in Splinter's eyes. "Put him on the table," he said. "Michelangelo, get the first aid kit."

Raphael stayed out of the way, but his sai stayed in his hands, a comforting, cold weight. Splinter and Donatello worked together on Leo, and now Raphael could see more echoes of Donatello in the sure movements, the clinical assessment of Leo's injuries. Without warning Donatello wrapped his arm around Leo's injured arm and twisted. There was a horrible, grinding click and then suddenly the arm was lying the right way. Raphael swallowed and looked away, shuddering.

"Dislocated," said Donatello to Splinter, who nodded and continued cleaning the lacerations across Leo's plastron. Splinter looked calm and collected, as he always did. How can you not be going crazy? Maybe he was. They were nearly done. Nearly done, and then there would be questions, and answers, and maybe he could stop feeling like he was going to explode.

When Leo's arm was strapped across his chest Splinter and Donatello carried his limp form down to the pit and lay him on a bench. When Donatello straightened, Splinter put a hand on his shoulder and looked up into his eyes. Raphael tensed, waiting for Splinter to find this Donatello to be a lie, to attack, but he only stared, frowning slightly.

It was Donatello who pulled away, putting a few steps of distance between himself and Sensei.

"So, my son," said Splinter. "I think you have a tale to tell us." Splinter sat carefully beside Leo, hands gathered in his lap. Mikey dropped down on the other side, eyes wide.

Raphael couldn't sit. He leaned on the pinball machine, arms crossed, not taking his eyes off Donatello.

Donatello also folded his arms, his gaze on the floor as if gathering his thoughts. "I'm- No." He shook his head at some inner thought.

Since when do you have trouble explaining anything, Donnie? "Just spit it out," Raphael snapped. "Or do you need some encouragement?" He would go over there and beat the story out of him in a moment if he didn't start talking.

Donatello glanced at Raphael, then transferred his gaze to Splinter. "I'm from the future. August, 2029, to be exact. I've come back to...fix some things that are broken."

"What things?" This time Splinter glanced at him, but Raphael was too worked up to care.

Donatello ignored him. "In my time, we lost." He started to pace, long strides taking him across the pit in a few steps. "The Kraang have conquered Earth. Humanity is hanging on by a thread. I'm here to prevent that."

The Kraang are going to win? No, he wouldn't believe that. They kicked Kraang butt all the time. "No way. We wouldn't lose to the Kraang. Every time they've tried, we've brought them down."

"Raphael." Splinter's tone was mild but the warning was clear. "How did they win?" he asked, his eyes on Donatello.

"They teamed up with the Foot. Shredder was the liaison they needed to infiltrate and destroy people in power who would have opposed them. With Earth's armed forces under their control, they started a war. And while we were busy killing each other, they took control. Mutated the world."

He stopped pacing, eyes on the floor, arms folded across his chest. "It's unlivable now, unless you have filters. Can't breathe the air. Can't drink the water without treatment." He breathed out in a hiss. "I'm here to stop it from happening. Because we can't fix it where we are."

A cold feeling was growing in Raphael's gut. "But if you're here, then where is Donnie?"

Donatello looked up at him. "We swapped places."

"WHAT?" Mikey was on his feet with a cry. Raphael charged down into the pit. "You sent Donnie into a future where the Kraang have won?" He snarled and brought his sai up. "Get him back!"

"I can't."

Sensei's hand landed on Raphael's shoulder. It was the only thing that stopped him launching himself at Donatello.

"It's a one-way trip, Raphael. I can come back, but I can't go forward."

"Why the hell not?" Sensei's hand tightened on his shoulder.

"It's complicated, Raphael. Do you really want me to launch into a technical explanation?"

"No, I want you to get my brother back." Horrible visions rose in his head. Donnie dumped in a toxic alien landscape, alone. He remembered the taste of the poisonous Kraang atmosphere, of choking on the air, losing consciousness. How would Donnie survive? Had the Kraang captured him already? "He'll die.

"I must share Raphael's concern," said Splinter. His voice was ice-cold with disapproval. "You have sent Donatello into a situation he is unprepared for. You may have doomed him to death. I am not pleased, Donatello."

Donatello faced them down. There was no regret in his eyes. "He's not alone. April is waiting for him." His eyes flashed. "I didn't have a choice. Travel only works if you can trade places with yourself, and you have to know your other self will be at a particular point at the time you need to transfer. Which is why you can't go forward. Because you can't predict where someone will be in the future."

Raphael felt Sensei's breath catch. His arm tightened around Raphael's shoulders. "So there is no way you can bring Donatello, our Donatello, back to us."

"No." Donatello looked straight into Splinters eyes. "You're stuck with me. And I have a job to do."

Splinters arms tightened across Raphael's chest as he flung himself at Donatello. "You-! Bring him back!"

Donatello turned away and walked towards the lab without a word.

Raphael raged until the door slid shut behind Donatello. Then he gave up fighting the steely grip of his sensei and sagged against him.

"Master Splinter?" Mikey's voice wavered.

"My sons," said Splinter. He reached for Mikey who wrapped his arms around Splinter. Raphael shook off his father's arm. He didn't want comfort. He slammed his sai back into his harness and stormed out of the lair. He needed space, and air, and something to smash his fists against until the image of Donnie in the hands of the Kraang went away.