Some TMNT stuff really isn't for little kids.
Bruce Wayne is a billionaire playboy socialite who is secretly a superhero known as Batman.
As a boy, Bruce Wayne's parents were murdered by Joe Chill, who attempted to steal his mother's pearl necklace. Since that fateful day in Crime Alley, Bruce from then on decided to avenge his parent's deaths by fighting crime and injustice that plagues Gotham City.
Batman/Teenage Mutant Ninja Turtles
Batman is contacted by a scientist who works at Powers Laboratories, who tells the Dark Knight about a break-in there. She describes a battle between the Foot Clan and the Turtles which resulted in the theft of an experimental generator. Batman vows to find the stolen tech.
Returning to the Batcave, he is able to deduce that both teams of assailants were skilled in the art of Ninjutsu, thanks to the scientist's description of the fight. His friend and butler, Alfred Pennyworth believes that the League of Assassins to be behind the burglaries, but Batman does not. The way the clan of ninjas moves does not match the League's modus-operandi, and there have been other thefts in Gotham City by these ninjas, all of which seem to be components in building something.
Batman then decides to stake out a Wayne Enterprises R & D facility because it is the next likely strike for the ninjas. Using some kind of hologram machine, Batman is able to fool the Foot Clan with false images of scientists who work there. Batman demands they give him the reason they have been stealing all this equipment. A fight broke out and Batman was victorious against the ninja clan. He interrogated one of the ninjas, who told Batman they merely "trying to get home". Batman then demanded to know about their metahumans, which confused the ninja. He tried to explain about these "turtles", but before he could, the ninja was killed by the Shredder, who warned Batman to stay out of his way or Gotham would pay the price.
A fight broke out between the five of them, in which Batman successfully used the Turtles' weapons against each them - he disarmed and tased Raphael, used Michelangelo's nunchaku to flip Donatello, and used Raphael's sai to break one of Leonardo's katanas. Leonardo begins fighting Batman in hand-to-hand combat, but Splinter intervenes and tells his children to leave, before making his own escape.
Bruce Wayne and Lucius Fox analyzed Raphael's sai, confirming that it came from another dimension. They also found traces of Raphael's blood on it, and discovered that the mutagen in it is becoming inert, which will cause the Turtles and Splinter to revert to their original animal forms. They also discuss that all the missing tech along with Doctor Naveen Khan's kidnapping, which indicates that someone wishes to build a portal to another dimension. The only missing component was a "resonance engine".
Bruce begins searching through the city for some trace of the Turtles and Splinter, only to discover that they have already found the Batcave and are roaming through it. Batman demands to know how they entered his cave, but Splinter surprises him by revealing he knows the superhero's true identity. Shaken, Batman listens to Splinter's recounting of where they come from and how they came to be there.
He decides to help the Turtles and Splinter make their way home, which he believes can be done that very night. The unified group storms Penguin's Iceberg Lounge, where Batman rescues Doctor Khan, and he and the Turtles take on the Foot Ninja. But just as victory seems assured, Shredder blows up the portal, and ensures another one will not be built by remotely killing Khan through a sub-dermal bomb.
In the next day, Batman began bonding more closely with the Turtles, sparring with Leonardo and even being coaxed by Michelangelo into eating pizza with them. When Raphael accused Batman of being a wealthy thrill-seeker, Batman convinced him to make a journey to Crime Alley with him, where he explained the death of his parents to the Turtle. More specifically, he explains that he fights crime in order to protect families, and that he's determined to help Raphael's family stay together and alive.
Shortly thereafter, Batman was called to a rooftop by Jim Gordon, who informed him - as well as Leonardo and Raphael - that Arkham Asylum had been taken over by the Foot. Before further steps could be taken, Leonardo collapsed from the waning mutagen in his system, and Batman had to quickly transport him back to the Batcave. As Casey Jones had entered Batman's dimension with a slingshot device, the Turtles now had the option of going home, leaving Shredder for Batman to defeat.
However, Batman soon discovered that Shredder and his new partner Ra's al Ghul were far too much for him to deal with, especially as they had utilized mutagen to turn the inhabitants of Arkham into deadly mutants. Batman found himself subdued by Poison Ivy and Mr. Freeze (now a mantis and a polar bear), when the Turtles unexpectedly arrived to save him. In the battle that ensued, he fought Shredder in his newly-constructed Intimidator armored suit, and managed to defeat him.
April O'Neil and Casey Jones returned through an experimental portal, allowing the Turtles to leave Batman's universe and take Shredder with them. Before leaving, Raphael gave Batman his mask as a sign of his respect for the superhero.
Back in the Batcave, Batman confirms that everything is going back to normal, and learns that the coming day is the anniversary of his parents' deaths. Rather than suffer alone, he decides to check the schematics of the Intimidator suit with his son Damian.
Batman/Teenage Mutant Ninja Turtles II
A ninja attempts to report to Ra's al Ghul about a new master of the League of Shadows, but is ambushed by Batman instead. He reveals that after Ra's al Ghul's humiliating defeat alongside Shredder, the League has decided that it needs someone with true strength.
Deducing who this new master is, Batman and Damian rush off to the underground Lazarus Pit, fighting their way past ninja man-bats, but are too late to stop Bane from using the Pit. At that very moment Donatello opens a portal between his world and Batman's, seeking Batman's help in becoming a better ninja and accidentally pulls Bane into his own world while he ends up in Batman's.
In order to pursue Bane, Batman receives part of an interdimensional transport device from Ra's al Ghul while Donatello and Damian obtain another part from Mr. Freeze. After assembling the portal device, Batman, Donatello and Damian all depart for Donatello's world, only to find buildings burning and chaos in the streets, as Bane has conquered most of the Foot Clan.
Batman is received warmly by Splinter and the other Turtles, but has to keep Damian in check as he repeatedly irks Raphael. He confirms that Bane is almost out of Venom, and that within twenty-four hours he will be immobilized by withdrawal, which is when they will strike. He, Leonardo and Splinter leave to spy on Bane, and witnessed him speaking to Baxter Stockman about the impending trials of a new batch of Venom.
After breaking up a fight between Raphael and Damian, Batman accompanied the Turtles to Stockman's laboratory, demanding to know where Bane is. It turns out to be a trap, as a Venom-enhanced Rocksteady and Bebop burst in, along with Venom-enhanced Foot ninjas. Batman tried to immobilize Rocksteady, only for the mutant to burst out of his chains, and was horrified when he saw Bane about to murder Raphael.
Back in the lair, Batman advised the Turtles not to use Venom in their attempts to take down Bane, and worked on strategies long into the night with Leonardo. Upon discovering Donatello's absence, he and Leonardo set out to convince him to stop using Venom and come home with them, appealing to him by saying that they need his genius. They eventually succeeded, bringing him home in time for Bane to declare his domination over the city.
To wrest control of the Foot from Bane, Batman broke into Riker's Island to find the Shredder, giving him his armor back and engaging him in a rematch fight. He, Shredder and Splinter then attacked Bane directly, knocking him out of the Statue of Liberty and fighting him alongside his allies Nightwing, Batgirl, and the Turtles. With Bane defeated and Shredder taken back into custody by the police, Batman and his allies returned to their dimension with Bane. He left behind a copy of his training protocol for his Robins in order to help Donatello.