A man named Bertram Smith recovers a lost miniaturized H-bomb from a crashed Air Force jet and sends a tape to police headquarters in Gotham, warning that he will blow the city off the map by midnight unless he receives $10 million in ransom. Only six hours remain till midnight, so there is no hope of evacuating the city.

Batman and Robin are called in on the case. The two heroes enlist the aid of Big Brill, Gotham's head gangster, battling their way into his penthouse retreat before they explain their mission. Brill calls in his division chiefs to listen to the taped message from Smith and try and identify the background noises to place Smith's location. One of the men, "One-Ear," places it on his turf. Investigating in the indicated place, Batman and Robin learn Smith's name and location from a TV repairman who fixed the tape recorder on which he prepared the message. Kids in the street outside Smith's boarding house give Batman and Robin a description of Smith's car and license number, and tell him that they broke his window with a ball in a street baseball game and were recently given a new ball by Smith.

Batman and Robin send the police track down Smith's car, but the bomb, found in the trunk, is tagged with a warning not to tamper with or remove it. Batman and Robin finally see Smith at the airport, watching the transfer of his ransom money to a plane. Smith tries to escape the heroes, but is struck by the plane's wheels as it taxis down the runway, and dies. With his last breath, he reveals that he planted the trigger to the bomb in a rubber ball and the Dynamic Duo hurry back to the boarding house, where they cut open the ball Smith gave to a kid named Skinnay, find the trigger, and deactivate it just in time.

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