03 AU where one winter when the turtles are kids, maybe five or eight, there's a collapse in one of the lairs, could be because of something like an earthquake, construction failure, etc. In the collapse, Donnie is crushed and dies. The pieces of rubble are so heavy the others can't move it to get the body and regretfully have to leave it.
The extreme cold preserves Donnie so it's still in perfect condition when Bishop and his team come to investigate reports of an "alien" being found in the sewers. They're even able to revive it with experimental procedures since the head was mainly left unharmed. A few of the less important organs, what appeared to be a shell, and the left arm needed to be replaced though.
The subject had no recollection of anything before being revived, likely a side effect of being dead for an extended time even if the effect was lessened by the cold. This means Bishop couldn't get any answers from it about whether there was more like it or where they might be. Though he could still stuffy it and test its capabilities.
The newly labeled L1038451 has an unexpected problem that messed with the data. The subject couldn't feel pain and they had no way of knowing if this was normal or another side effect of it temporarily expiring.
Bishop sitting in his lab wondering what he could do next with this creature when he notices it running across the doorway of his lab. He immediately gets up and chases it down, which is surprisingly hard considering its small size.
Bishop demands who let it out of its cell when he notices one of the tools from an earlier experiment in L10's hand. When did it even grab that? Bishop looked from it to L10's face and saw the most cheeky smile. Why this little- Bishop is going to make sure it can never leave its cell again.
It leaves its cell on its own again less than two days later. Making sure to pull on Bishop's pant leg and everything so he knows.
Bishop doesn't understand! No matter what locks or security system he puts up it somehow opens them! At the same time making sure to erase the camera footage so he doesn't know how. Then it just seems to seek him out to show off its win. Worst of all it started greeting him with this irritating "Guess who, Bippy?"
As Bishop is grumbling he notices hasn't seen L10 in a while. Why doesn't he believe it's because he's finally contained that thing?
Bishop searched the facility and found L10 in his cell. With a bunch of parts that weren't in there the last time Bishop put him back in. Is- Is he making a drone in there? How does it know how to make a drone?
Before Bishop realizes it he's sitting in the cell with L10 watching it build and sometimes offering tips and ideas. When it's finished and L10's eyes light up with joy Bishop almost feels… pride?
That feeling is short-lived as L10 gets up and runs out of the cell giggling, locking Bishop inside behind him.
L10 does eventually learn how to hack into Bishop's bank account and order stuff to be sent there. Bishop's looking so confused when he sees purchases under the name "Elton". Elton? El-ton. L-10. L10!
