Some TMNT stuff was always meant for grownups.
Appearing in #9
Following a lead on the whereabouts of Splinter, Leonardo flies the Triceraton Aircar into Midway City. While reading the city's local newspaper, he's attacked by a mutant bat that was sent by Splinter to find and attack Leonardo. Leonardo fights the bat, taking the battle down to the streets. As he is about to stake the bat to death, he is stopped by superhero Knight Watchman. Watchman tells Leo that since the mutation is spread by rabies, a rabies vaccine also reverses the mutation, as he injects it into this mutant bat. Watchman then reveals to Leo that they had met before - and Leo realizes that Knight Watchman was who he knew as Galahad.
Knight Watchman tells Leo that once the "lab boys" determined that mutagen was involved, he tried to reach out to Splinter and the Turtles for help, but he was unable to get in contact. Leo explains how Splinter is involved in all this and how he was somehow mutated further into a mutant bat by Dr. Wu's synthetic mutagen.
Leonardo unfortunately comes to his aircar and finds that it's been stripped for parts. Luckily, Knight Watchman had already come across the goons doing it and tied them up, although Leo doesn't know how to put the pieces back together.
Back at Knight Watchman's civilian residence, he and Leo discuss his identities as Galahad and Knight Watchman. Jerry Randall operates as Galahad during the day and Knight Watchman at night, taking the latter mantle from his retired uncle Reid, as sort of a good cop/bad cop routine.
Leo tries to contact Splinter on the astral plane, but as he meditates, suddenly a mutant bat and a bunch of regular bats crash through Randall's window. Randall fights off the bats and just as he is about to inject the mutant with the antidote, the original mutant bat barges in. This mutant bat flies out Randall's window and reverts back into a human, but is too far for Randall to reach. Fortunately, Leo breaks meditation and rescues the man with his kaginawa. The other mutant bat then escapes, but Knight Watchman had planted a tracking bug on him in hopes that he would lead them to Splinter.
They follow the trail to a closed-down club called "The Mine Shaft". Inside, they find scores of mutant bats. Knight Watchman gasses the place and injects them with the antitoxin, while Leo handles Splinter. Splinter tries to fly away, but Leo hitches a ride on his back. Leo manages to inject Splinter with the vaccine, but he is tossed into a tree. Knight Watchman catches up to him and Leo laments their failure, as Splinter flies out of town, seemingly unphased. Knight Watchman says that they saved the city and he's proud of Leo, as the turtle drops out of the tree onto his head.