In a remote house, the arch villain Joker thanks his pet hyena Giggles for bringing him the paper. The Joker sees an article in the paper and exclaims he's going to be rich.
At Wayne Manor, Dick Grayson and Bruce Wayne are playing a game of pong and with Dick Grayson losing; he has to cook for the rest of the week with peanut butter and sardine pie first up. The Bat signal flashes and the two slide down poles to emerge in the Bat Cave.
Badman senses something is wrong and Bat-Mite appears moments later. Batman says Bat-Mite was told to stay on his own world and Bat-Mite says all he wants to do is help before he disappears. Batman and Robin leave in the Batmobile moments later to see what Commission Gordon wants with Bat-Mite hitchhiking on the car's back bumper.
At police headquarters, Commissioner Gordon shows Batman and Robin a video sent to him that shows Joker declaring he'll pull off the greatest robbery at precisely seven that evening and leaves a riddle about where and what it is. Batman and Robin leave with Bat-Mite close behind.
In the Bat Cave, Batman and Robin try to figure out the riddle but the Bat computer proves no help. Batman sees an article about a scientist unveiling a new car prototype which runs on water rather than gas that evening and deduces that is the Joker's target. Batman alerts the Commissioner while Bat-Mite tries to start the Batmobile on his own but Batman stops the car remotely and tells Bat-Mite to stop trying to help.
Batman, Robin and Commissioner Gordon, in a surveillance van disguised as a laundry truck, await Professor Irwin's arrival and unveil his prototype. Irwin arrives but Batman sees something is wrong and cameras pick up the fact Irwin is wearing a Joker ring. Moments later, Joker reveals it is him disguised as Irwin and drives away in the prototype. Batman has to delay pursuing him to save Bat-Mite's life.
The real professor is found up and says they have to get his prototype back as it can explode with a force of a nuclear bomb though the professor has partial amnesia and can't recall exactly what the circumstances are.
Joker broadcasts over Gotham's televisions from the police parking lot and escapes by flooding the police cars before speeding away and vowing to sell the prototype to the highest bidder. Batman tries to stop him with a temporary spring barricade but Bat-Mite tries to help and Joker gets away in the chaos.
The professor says that he recalls that if salt contaminates his car's power unit, it will start converting all nearby water to energy until it explodes. Batman and Robin head to the harbor, where they figure the Joker has the car. Robin looks around the dock and Bat-Mite tries to help but is caught by Giggles and brought to the Joker inside a nearby ship. Bat-Mite tries to teleport away but steel stops his powers from working.
Robin is caught by the Joker in a net and the Joker tries to lure the Batman into a trap pretending to be Robin. Batman, however, realizes it is a trap and captures the Joker in the Bat Gyro before freeing Robin and Bat-Mite and stopping the car from detonating.
Back at Wayne Manor, Bat-Mite eats up Robin's pie and disappears with a queasy look on his face moments later.