I am an engineer, not a doctor. But the body is just one big machine, isn't it? Confronted by the mangled meat and bone that is a severed limb, I repeated that to myself as I worked.

"Just a machine," I whispered, capping an artery—just like a very small fuel line. "Just a machine" as I filed down the bone shard—just like grinding steel.

but it isn't a machine, it's my brother

I shut that thought off, compartmentalizing it away so I didn't have to think about the pain, the shock, the fear he must have felt. Leonardo didn't even twitch anymore, so lost in the numbing haze of morphine painkillers.

Like putting a machine in sleep mode.

And as I followed the field medicine guidelines from the internet—I had never studied for this, and how stupid was I for not preparing for this obvious eventuality? Stupid, stupid—I was struck by a terrible realization.

Yes, I could fold over the skin, stitch the wound, wrap a stump, and prepare molding for prosthetics, for all the good a fake hand would do for a ninja. Useless to climb with, to fight with. Leonardo would be crippled, kept from the street warfare he could only long to return to. I know him too well. He wouldn't want to leave the fighting to us.

But what choice did I have?

I looked around my laboratory. On the endless shelves, in boxes and scattered loosely on the floor lay all the relics of years of space adventures, explorations on other planets and thieved resources from intergalactic ships. I had piles of Utrom machinery and pieces of Triceraton weaponry. I had studied these for months. I was surrounded by a room full of alien tech.

And my brother, laid out on a steel table like the most intimate of test subjects.

In my mind, arteries and veins connected to thin conduits, nerves lit up the most delicate yet tough relays that directed joints made out of gears and muscles of carbon nanotubes.

I didn't have to consign my brother to stiff plastics. I could give him a new arm that would be a hundred times stronger—more flexible, more agile. All I had to do was reinforce the shoulder...

No. The shoulder was too weak. I'd need to cut higher, remove the damaged tissue and create a starting place on my own terms.

I cauterized the existing wound—I'd be removing three or four inches above it anyway—and then hooked up an intravenous line for a morphine drip.

If I worked fast and gave the new hand just three fingers, sheathed the limb in a kind of smooth sleeve to hide the gear and wetwork, then my brother wouldn't even suffer the dysmorphia that amputation can bring.

"The worst part's over," I whispered to him. He couldn't hear me, lost in a pleasant haze, but I told him again anyway. "It's going to be okay now."

It was more than okay. It was perfect. Sublime.

Over 1345% percent increased output than his organic arm. His cybernetic hand crushed concrete to powder without any noticeable resistance. His arm could lift his entire bodyweight in one-arm pull-ups without fatigue.

He used it like a pro, even though I could tell that he wasn't quite comfortable with it.

I sometimes found him tugging at the seams, the thin stitches in the polycarbonate weave that acted like skin. Sometimes he held his new arm in his organic hand, feeling the mechanisms at play. His curiosity was endearing. I'd shown him how it works on the monitor, showing off the 3D model. He didn't understand the details, the sheer depths of genius this took—none of them did—but his wonder at seeing the alien gear at work was satisfying.

My brother had the utmost faith in me. The looks he gave me bordered on nothing less than worship. His little techy brother could move mountains with a screwdriver.

Master Splinter, though...

He gave my work wary side-eyes, holding himself aloof as if the hand might suddenly lunge of its own will. Ridiculous. The artificial tendons were connected to a skeletal structure of alien steel controlled by sensor relays wired directly into Leonardo's nervous system. It was completely under his control. But Splinter acted like the arm was evil.

In the kitchen at 5 a.m., late for me and early for him, we both waited for the kettle to boil. Between us, we kept a clear personal space, standing respectfully apart. His disapproval was obvious in every move, every glance in my direction. His tail twitched, and I felt his resentment.

"Would you rather he have a plastic arm?" I asked.

Splinter looked at me with wide eyes. "What?"

"'Cause that was literally the only other choice," I said. Too forcefully, too quickly. It didn't matter. I had to push it out before I lost my nerve. "Give him a glorified mannequin limb or give him something he could use."

"My son, I did not—"

"I've seen how you look at it," I said, refusing to let him get away with those looks. "Like it's evil. Like it's not natural. Of course it isn't natural—it's better. You all thought he'd be destroyed losing it and I fixed it."

"I am sorry if I made you think I disapprove of your work," Splinter said, trying to...I don't know what he was trying to do. "I am grateful for your work. It is as you say. I was afraid Leonardo would not recover in spirit, let alone in body."

But.

Splinter didn't say anything for several seconds. My heart pounded. But? Come on, there had to be a reason for his glares. A rhythm built in my brain, pounding with my heart. Come on. Just say it. But—

The kettle sounded.

Splinter served his tea and left the kettle, left the kitchen, left me.

Repair and upkeep were easy. I made Leonardo look away from the arm during maintenance. So far he hadn't reported any phantom pain and I wanted it to stay that way. He seemed to have completely accepted the arm without any emotional backlash. The arm served without any malfunction for months, a year. Two years.

So I had high hopes after the explosion that took his other arm and right leg.

It was Mikey's fault—Michelangelo's and Raphael's. They followed Leonardo into a crashed alien ship that was already counting down to self-destruct. He was retrieving the crystal core for me, but they hadn't let either of us know they were going in, too. Leonardo was on his way out when they surprised him at the door. In pushing them clear, he lost the split second that would have let him land safely as well.

I tried not to blame them. At least they got him home alive. Their guilt made them attentive lab assistants. And the crystal core was intact, which meant I could use its energy to power the cybernetic arm and legs I had created.

I'd been preparing new limbs for this eventuality. I didn't want to be caught so off guard again as with the first arm. And these limbs were much more advanced. Two years of improvement and streamlining.

This time, his after-surgery care required therapy. It was hard to walk on one cybernetic limb and one organic limb without going too fast, too slow, or misjudging a step. Even harder when he had to hold onto the two railings I made him with two cybernetic hands. But he was a good test subject. He tried hard and took correction without judgment.

I had to rework the skin sleeves, though. I thought they were perfect, but then he confessed to me that he couldn't really feel anything through them. His arms felt numb and he couldn't tell where his new foot was falling. This wasn't a problem with just one artificial arm, not when his machine hand could "feel" due to where the steel landed, but now that he was half mechanical...

I should have realized that. I took a few days weaving extra sensors and relays into the skin like thread, anchoring them to circuits at key points. They wouldn't be as sensitive as his real skin, but at least now, with a little voltage that carried through the relays to his nervous system, Leonardo could feel "himself."

And the output was incredible. He could lift hundreds of pounds, move faster than Shredder's robots, punch through Stockman's m.o.u.s.e.r. units. And he moved like a spider, twisting his limbs at impossible angles to crawl along pipes and vents. He could even climb sheer walls. His fingertips could punch through brick or hook into steel without slowing him down.

He is half my machinery, and my creation works perfectly.

Alone in the lab, whispering, Leonardo told me that he sometimes dreamed he was short circuiting or falling to pieces. That his limbs suddenly lost power and he lay like a doll with its strings cut, hoping that I would find him.

That sometimes he felt short, sharp shocks where plasticene met bone.

I also added an independent supply of morphine to his design. I didn't want him addicted, but so far he'd shown no increased resistance to the drug, and I didn't want him in pain, either.

I removed his last organic limb simply because he said it was making him feel lopsided.

Something had changed in the lair. I didn't announce this alteration. Neither did Leonardo. We simply wrapped all four limbs in a sheathe of similarly colored material that closely mimicked his original color. It was slightly off but he looked normal. He acted normal.

Michelangelo and Raphael didn't dare criticize. Neither of them tried to keep up with him, either. No one attempted to spar with Leonardo after that. He trained on his own, learning the new body I made for him.

Splinter carefully kept his criticism to himself. I know he was thinking it. It didn't matter. He left me alone with my experimentation. On the rare late nights we saw each other in the kitchen, he didn't comment on the dark circles under my eyes, the signs of strain as I constantly re-engineered my designs.

Leonardo's only weakness was the shell and plastron, the points of connection at his shoulders and pelvis. If I could simply go in, exchange his internal structure for something manufactured, something machined in my laboratory, I could remake him into something much more durable.

I caught Leonardo staring at himself in the mirror, running a hand along his new parts. I didn't know if he was trying to get a sense of himself, to feel what he was becoming. What I was turning him into. He always did think with his body. A kinesthetic learner. To have his body altered in such a fashion must have been disorienting.

He lay on the steel table and watched me work. He was so still that I didn't have to strap him down, but it was good lab procedure to do so. He told me when it hurt. He told me when he could stand the pain and when to keep going. He sometimes cried and said to stop as I inserted the last wires into live nerves. He forgave me when I turned on the morphine drip.

I began to wonder if I could replace his shell.

I could.

I did.

Slowly, to reduce the shock on his body, I replaced more.

So much in his core became redundant if he didn't have to eat. Nutrients were required solely for the brain and spinal column. I could calculate those precisely.

He still looked like himself, still like Leonardo, but I have reworked him into something so much more powerful. Capable. Nothing had defeated him ever since. Nothing ever will.

I pinned him in my lab—arms screwed above his head, legs bolted into the floor—and locked his head into a steel framework designed to keep him still.

"This is going to feel weird," I warned him. "I need to access your cerebellum and brain stem, so you're going to feel pressure back here."

"You aren't going to knock me out?" he asked.

His voice shook. After all this time, he was still afraid of surgery.

"I need you to be awake and responding," I said. "Just listen to the sound of my voice and you'll be okay."

He couldn't nod, so he murmured something and fell silent.

The benefits of his artificial shell, aside from the aesthetically polished surface, was that I can more easily access his spinal column. I used an Utrom surgical device to help—it had four magnification lenses and microtools that let me alter my brother at the cellular level. I quietly stated everything I was doing so he could listen to me, although I was sure he had no idea what subcutaneal neural implantation meant. But when this was over, his new systems wouldn't need any kind of outside recalibration—his brain would do that for us. And he would better feel the edges of his cybernetic body and adjust accordingly.

When I accessed his cerebrum, though, and begin insertion, he told a quick breath and froze.

"Are you okay?" I asked. "Did that hurt?"

Impossible. Brains do not feel discomfort.

"I just remembered being with you on the Hudson bridge." There was a note of surprise in his voice, almost like he was seeing the memory replay. "We're watching the clouds."

"I...don't remember that," I said, returning to work.

"Neither did I," he said. "And...Mikey was with us. Raph. We were just sitting and enjoying being out. I..."

It must have been the insertion of wires triggering the memory. I'd never heard of it happening from insertion into the cerebellum, but the trauma of it could have been the culprit.

"There was sunlight," Leonardo whispered. "Wind. I could feel it. I..."

He stopped. I finished the insertion and added the insulation that would keep these points safe, fusing them down to the bone.

"I don't want to be this."

I froze.

"I don't want to be this."

I didn't have to move around him to see what he was looking at. In my most poorly thought out decision ever, I'd placed the lengthwise mirror across from us. I'd meant to see his facial expressions, to make sure that he was awake and all right. But he saw only himself—nevermind the craftsmanship of his new parts—he saw little more than a head on burnished steel. I had removed his skin sheathes to make sure the machinery worked smoothly with this new upgrade, and all he saw was the gleam of plastic and wires.

His breathing turned erratic. He yanked at the pins holding him in place, and—afraid he'd harm himself—I cut his power output to 2%. It stopped him from moving, but now panic started to sink in. It struck me now that he'd never cried over the loss of himself, and maybe now it was finally sinking in that we'd reshaped him so drastically.

"Don...Don..."

He sobbed my name over and over. No melodramatic crying that he was a monster, that he wasn't supposed to be alive. We were already freaks, after all. But he couldn't feel the way he used to and he couldn't recognize himself in the mirror, and for him, that must have been frightening.

A chemical mix of benzodiazepine and scopolamine calmed him until he rested quietly, nodding when I asked questions. When I finally released him and helped him down, he moved only to obey my commands.

I made gel capsules with a mix of opioids. Much easier to keep him calm and compliant. He now takes his doses out of my palm.

His new eyes shine like stars in the dark.

My final modification, at least for now. If I activate his omnidirectional weaponization, he can unsheathe four blades, one from each limb. When he has practiced enough, he will be a whirlwind of death from any direction. For now, he has to learn how to walk with them.

I unveil him to absolute silence. Michelangelo, Raphael and Splinter all stare and back away, their mouths open, as my creation comes forward from the darkness, sharp steel points stabbing the floor like spider legs, his gaze always on me. So full of faith in his little brother.