Mayor Grange is in an election against millionaire Maximillian Zeus, who is losing after revealing several strange ideas like his idea of armored policemen/gladiators. Bruce is introducing Grange at his victory speech when Zeus announces that he refuses to accept the election results and will rule the city from his giant aircraft, New Olympus. Bruce summons the Batmobile while Zeus fires a blast that disintegrates City Hall.
Barbara is in the crowd with her father and ducks away to become Batgirl and go after Zeus, who is announcing his "decrees." Batman heads up in a backpack glider but is shot down and swings to safety. Batgirl gets to the rooftop while Batman plans to meet Zeus on his turf and drives off. Batgirl sees him going and wonders what is going on, but boards the airship and dodges the robot gladiators. Grange and Commissioner have no choice but to wait for Batman, who is at the Batcave preparing his newest aerial vehicle, the Batwing.
Batgirl prepares to shut down the airship's power core while Zeus prepares to destroy the Gotham Bay Bridge to keep the citizens from fleeing. He activates the Thunderstorm cannon while Batgirl starts pulling cables. She cuts power to the firing mechanism but the cannon itself continues to power up. With seconds left Batgirl manages to cut the main power supply. Before Zeus can track down Batgirl, Batman approaches in the Batwing and fires missiles, which are deflected by New Olympus' defense systems. Batman disables the defense systems so Zeus unleashes his "harpies" – anti-aircraft attack drones. Zeus fires on him and damages the Batwing, while Batgirl sneaks by Zeus' soldiers and deactivates the weaponry just as Zeus prepares to blast the Batwing from the sky.
Batman lands the Batwing atop New Olympus but the gladiators stun him with electrorods and take him before Zeus. Zeus prepares to blast him out of existence but Batgirl announces herself over the loudspeaker and threatens to destroy the Thunderstorm's critical component. While Zeus' men search the ship for her, Barbara takes a brief call from her father but Zeus locks onto her frequency and closes on her location. Batman manages to escape and take out the guards, while Batgirl stays ahead of the guards until Zeus corners her. Batgirl destroys the component and he starts blasting away – he brings her down but Batman arrives and knocks him into a control panel, damaging the ship's antigrav unit. New Olympus starts to plummet and the crew evacuate.
Batgirl heads for the cockpit to land the craft safely but Zeus refuses to yield and he and Batman fight. Batman knocks the villain out through the side and descends on a parachute while Batgirl tries to steer New Olympus into the harbor. She succeeds and gets clear, while Zeus is forced to see his intent destroyed.
At the Batcave, Bruce is working on the Batwing and with some prodding from Alfred realizes she's ready. Later Batgirl responds to the Batsignal and finds a case holding a Batwave, utility belt, and batarang.