During a stormy night in Gotham City, an antique dealer prepares to close his shop. As he is closing his door, a woman forces her way in. She is Harley Quinn and using a prank music box, she sprays the shop owner with laughing gas. As he laughs uncontrollably, the Joker enters. The Joker looks around and spots a statue of a green dragon. He steals it and heads out.
Later Detective Harvey Bullock looks over the crime scene. He is confused as to why the Joker would steal the statue and ignore the other valuables. Commissioner Gordon is also puzzled because word of the street is that the Joker is looking for cash. Batman arrives and agrees with the two policemen. He examines and later analyzes a bit of green residue that he finds on the statue's base. Batman discovers that the statue is emitting a low level of radiation. He tells Alfred that he is heading to Metropolis.
Sometime later, Lois Lane sits aboard Air Force One. Suddenly, the aircraft that is carrying the president is taken over by hijackers. Soon Superman arrives to save the day. He enters the plane and deals with the hijackers.
On the ground in Metropolis, the president thanks Superman. After the president leaves, Superman apologizes to Lois for shaking the plane up. Lois tells him that she is getting used to it. As Superman is about to leave, Lois tries to ask Superman out. Unfortunately, a bank robbery interrupts her and Superman heads off.
In his Penthouse apartment, Lex Luthor reads the newspaper and grows angry at the press's coverage of Superman. Luthor tells Mercy to prepare his limo. Just after she responds, she is knocked out by Harley Quinn. Luthor heads to the limo and discovers that it is being driven by Harley. She heads into the street and picks up the Joker. The Joker enters the car and tells Lex that he wants to make a deal. The Joker says that he will kill Superman for a billion dollars. Luthor laughs and asks the Joker how he expects to kill Superman when he can't even deal with Batman. The Joker reveals that he has an ace up his sleeve: a green statue made out of Kryptonite. Luthor agrees to the Joker's plan.
Later Lois and Clark await the arrival of millionaire Bruce Wayne at the Metropolis Airport. Bruce has come to Metropolis to oversee business negotiations with Lexcorp. When he steps out of the plane, Lois is struck by his good looks. Bruce steps up to her and begins a conversation. He offers her dinner and an interview. She accepts and Bruce heads to his limo.
Some time later, the Joker confronts a mobster named Ceasar Carlini. The Joker tells Carlini that he needs a place to stay. Carlini orders the Joker killed, but Harley Quinn knocks out his gang. The Joker uses laughing gas on Carlini and throws him out. He tells Carlini's gang that they have a new leader.
In a hospital Superman sees Carlin in a laughing fit. He realizes that the Joker has come to town.
The next day, Bruce Wayne and Lex Luthor oversee a new robot prototype being tested in a canyon. Wayne wants the robots to be used for unmanned space travel. Lex on the other hand believes that the robots have potential military applications. Wayne disagrees and informs Lex that according to their deal, the robots cannot be used for anything without his approval.
That evening, Bruce takes Lois out for dinner. They talk about Superman, but Lois tells him that she doesn't want to talk about Superman anymore. He agrees and the two dance.
Sometime later, Clark Kent asks his contacts to keep an ear open for word on the Joker. At the same time at a dance club, Batman begins his own investigation. He confronts a former gangster named Binko. He tells Binko that he knows that his former boss Carlini has been replaced. Batman fights off a number of Binko's bodyguards. He grabs Binko and pushes him against the wall. He tells him that he wants to know the whereabouts of the Joker. Batman begins to press Binko, but Superman arrives and tells him to stop. Superman grabs Batman's arm, but using judo, Batman throws Superman across the room. Superman is surprised, but knocks Batman over. Using his X-ray vision, Superman discovers that Batman is Bruce Wayne. He tells Batman that he does not want vigilantes in Metropolis. Batman says that he'll leave after he finds the Joker. He tells Superman that the Joker is out to get him and that he has twenty pounds of Kryptonite. Before Superman can respond, Batman disappears.
Superman heads home. He goes to his apartment and changes back into Clark Kent. He gets a phone call from Lois who tells him that she has a date with Bruce Wayne. As he chats, Clark notices a tracking device on his cape. Clark looks out the window and sees Batman watching him with binoculars. Now both of them know each other's secret identities.