The lab door closed behind him. Don paused on the threshold for a moment, steeling himself against the wash of memories. Voices rose on the other side of the door; Splinter's measured tones, Michelangelo's agitated voice, and another, quieter voice. Leo must be awake.

He pushed the voices aside and headed for his desk. It was a mess; books, glassware and tools piled haphazardly. In a clear space in the middle lay an open notebook. He scanned the page. Notes on developing retromutagen. Well that was a waste of time. He grabbed a pen, scrawled DON'T BOTHER across the words, then swept the whole mess off the desk and on to the floor.

He reached over and dragged the laptop into the clear space. Don't touch my computer, Mikey came up on the screen, with an entry box for a password. Damn. Something else lost to memory. Oh well, he'd just have to hack in. He rebooted, got into the root directory, worked his way around the primitive file system until the profile was clear, then booted again. Too easy.

He lost himself for five quiet minutes setting up automatic web searches. Such a primitive system, compared to the lightning-fast Kraang technology they worked with now. When the searches were done, he straightened. Nothing to do now but wait and see what came up, whether it would match the events he was expecting, or whether he had done enough, now, to change things. He doubted it.

His gaze wandered across the lab. So many projects. One of the patrol buggy sidecars was in pieces. He couldn't remember why he'd pulled it apart. The Shellraiser lurked in the corner, bright colours against the grey walls, a panel on the side open and spewing cables onto the concrete floor.

He made his way over to it. It looked like he had been working on the camera interface. Another thing he had forgotten. Not surprising, considering. He ran his gaze over the brick-like shape. Such a crude design. His mind automatically presented a number of improvements. He pushed them away. He wasn't here to play.

Still, not a bad effort for a fifteen year old with only scavenged parts and crappy tools to work with. The pride he felt surprised him. He hadn't thought there was any pride left in the shattered remains of his ego. He reached up a hand and patted the steel body.

"You look like you're saying hello to an old friend. Or a pet."

Don spun around. He had been so lost in his memories he hadn't heard the door open. Leo stood in the doorway, his face pale, the white bandage holding his arm to his chest stark against the green skin. But he stood tall, chin up, observing Donatello with a measured gaze.

"Something like that," said Don. Was there ever a time when Leo wasn't in control? He tried to imagine what Leo would be like at thirty-two, his youth tempered by responsibility and hardship. But he couldn't. You'll forever be sixteen in my head, Leonardo. He steeled himself against the jolt of grief that washed over him. Still sharp, after all these years.

Leonardo walked across the lab, stopping a few paces away. Don could see his eyes searching, looking for something of his brother in the person before him. I wonder what you see. Would you have thought I would end up like this?

The observation over, Leo looked up to meet Donatello's eyes. "Master Splinter says that you and Donnie have traded places."

"That's right." Those damn eyes. He forced himself to meet the gaze, not to look away.

"And that you can't bring him back."

Don nodded slowly, not trusting his voice.

A quick intake of breath, the blue eyes narrowed with concern. "Why?" There was no anger there. Not like Raphael, whose every question held accusation. He could see the struggle to listen first, to not react.

"If I explained, would you understand?"

Leo thought for a moment. "Probably not. All right. Then tell me why you are here. What's going to happen?"

"What's going to happen is that we- you, are going to lose."

"Because of Shredder teaming up with the Kraang?"

"That's one problem, yes."

Leo put his head on one side. "What's the other?"

"Others. I could be here all night explaining." And he really didn't want to say it, to face those blue eyes and say you should have died tonight.

Leo shrugged. Pain flashed across his face, then was gone. "I don't have anything else to do."

"Fair enough." Don walked past him to the desk and opened up his searches. "There are hundreds of events that happened that helped to defeat us, but we've pinpointed a few of the major points that, if prevented, might be enough to change the path of the future."

Leo leaned in to stare at the list of words. He pointed to the top of the list. Oruku Saki and Karai and then New York Defense Force. "Shredder and Karai? What defense force?"

"In a few days, the Kraang are going to release mutants across the city. These aren't Earth-created mutants. These were made in Dimension X. They're basically super-soldiers, sent here to terrorise and destroy. Shredder funded a defense force, run by Karai. Except the 'defense force' is just a front for indoctrinating people, sending them to Dimension X to be mutated into more super-soldiers, then sent back here to fight."

Leo frowned. "But why? Why fight each other?"

"Because they can destroy key facilities, kill important people, all under the guise of 'defending the city'. New York is the first. After they prove the success of the defense force against the mutant soldiers with a couple of staged fights, they set up defense forces in key locations all over the world."

He heard Leo's breath catch. "And then-"

"And then it's one short, fast ride to the downfall of humanity. So we have to win here. Now. By whatever means possible."

Leo stood silently for a long while, frowning, eyes on the desk. "And you're here to help us stop them."

"Yes."

Leo looked up and met his eyes. "And Donnie? Will he be all right?"

Don shrugged. "Probably not. Who knows?" I hope so. Because I can't go home without him. But he wasn't going mention that, to raise hope when there was so little chance of this ever being fixed. Or maybe it was his own, desperate hope he was steeling himself against.

Now there was a flash of anger in the blue gaze. "That's my brother you're talking about. My brother that you've sent into danger."

Don looked back, drawing on all the cold, dark regret in his soul to face down that hurt. "That's ME I'm talking about, actually. I sent myself into danger. I'm paying for this with my own life, nobody else's. You of all people, Leo, should understand sacrifice."

He turned his back on Leo and focussed on the computer. After a moment he heard the soft pad of feet on concrete, then the sound of the door sliding closed.