Meet Bruce Wayne, a wealthy swinging bachelor in his mid-twentys. But Bruce isn't a normal multi-billionare; he has an alter ego of the greatest crime-fighter in all of Gotham. Bring on The Batman! When crime strikes - Batman's on the scene. Mob boss Rupert Thorne and his thugs are hiding out after stealing some cash - until they're interrupted by The Batman. After cleaning the crime, Bruce heads back to the Batcave where his butler, Alfred, has thrown him a surprise, a cake for his third year anniversary of donning cape and cowl.
The next morning, Bruce and Alfred watch the news where they're speaking about The Batman. Alfred reminds Bruce that he must go to the Gotham Gators basketball game that evening. Reluctantly, Bruce accepts, although he was planning to do work in the Batcave. At the police station, Chief Rojas introduces a new partner to Detective Bennett: Detective Ellen Yin of Metropolis. Rojas demands that they bring him The Batman. Back at the Wayne mansion, Bruce is ready to put on the bachelor party-hat, but Alfred assures him that he's never far from the Batcave with an invention dubbed the Bat-Wave.
At Arkham Asylum; a prison for dangerous criminals, a bomb tech checks the jail cells to see that everyone's there. He notices someone in a jail cell that should've been vacant. The bomb tech goes in to check out who's inside, and it's none other than the Joker! Joker gasses him, and then releases all the prisoners. Back at the police station, Detectives Yin and Bennett learn of the escape in Arkham Asylum. They immediately get on the scene. The Bat-Wave also goes off, so Bruce is forced to leave the Gators game. Bruce puts on the Bat costume and heads to Arkham in the Batmobile. As the police arrive at the prison, they find a giant jack-in-the-box near the entrance. The thing explodes, while The Batman watches from above. He goes inside the prison, only to find the gassed bomb tech. Joker mocks him from directly above. The Batman questions him, but Joker doesn't take him seriously. Joker attempts to gas Batman, but Batman quickly throws a batarang, knocking the gas out of Joker's hand. Batman and Joker begin fighting.
Outside the prison, Yin and Bennett arrive. The police are rounding up patients and doing other work, so Yin and Bennett take a swim to Arkham. In the prison, Joker manages to escape from Batman, so Batman leaves and takes the gassed bomb tech with him. But before he can make a safe escape - Detective Bennett spots him. Batman quickly uses the grappling hook to escape.
Batman goes back to the Batcave, so Alfred can care for the gassed victim. The next day, Bruce thinks of a plan to find Joker. He must go to Joker's old hideout and get the gas. Detective Bennett comes down to the Wayne mansion to talk to Bruce. He tells Bruce that he saw The Batman last night and that he thinks Batman is actually helping the law, but that he must stop him.
Later, Batman goes down to the Monarch theater - Joker's old hideout. Unsurprisingly, Joker's there, getting ready for his master plan. Batman demands the gas, but Joker refuses to give it to him. Instead, he takes off in his hot air balloon - the balloon is filled with gas and he plans to pop it over Gotham! Joker is getting closer to popping the balloon, but Batman comes to the rescue. He fights Joker on the hot air balloon and nearly falls, but with his quick wit, he moves the balloon away from the statue that would pop it. He calls for the Batbot via Bat-Wave and takes the balloon under water and pops it there. He escapes using the Batboat, and also takes a sample of the gas to make an antidote. Bennett and Yin arrest Joker and send him to Arkham Asylum.