In order to guide shelling from a submarine off the coast of California, Axis agents Frederick Hoff and Yamanito plot with American Jerry Silva to plant a time bomb on a bus that travels past an oil field on its way to San Francisco. It is Silva's assignment to ensure that the bus is at the proper point when the bomb explodes, illuminating the target. At the bus station, while Silva waits to plant the bomb, the passengers begin to assemble. Among them are Marine Sergeant Ryan; Henry Dipper and his bossy wife, who are celebrating their golden wedding anniversary; lecherous salesman Nick Stoddard; and two old maids, who while away the time by clucking over attractive Reba Richards' attempts to borrow money for a bus ticket. Silva is also watching her efforts and offers Reba a bus ticket if she will agree to carry his briefcase, which, unknown to her, contains the bomb. Ryan is suspicious and asks to examine the briefcase, but Silva manages to switch the bomb to Reba's suitcase before the search. Danny, a young man eloping with Betty, sees the switch, and Silva knocks him unconscious. When Betty notices he is missing, she rouses the police and Danny is found. Meanwhile, the bus is held until all the bags are searched. Danny reveals what he saw, but once again, Silva removes the bomb. When he tries to hide it on the bus, he is seen by an undercover agent posing as a panhandler. The agent arrests Silva, but Hoff, who is observing Silva, knocks the agent unconscious and Silva enters the bus. While he is there, Reba gets on the bus to wait and Silva sneaks the bomb back in the briefcase. Nick has followed Reba to the bus and when he makes a pass at her, she slaps him and leaves. Silva kills Nick and leaves the bus unseen by Reba, who has been flirting with Ryan. After the body is found, all the suitcases are transferred to another bus. During the transfer, Silva sneaks the bomb into a box of oranges that Henry brings inside the bus and they finally depart. When the injured agent is discovered, he tells Quinn, the station policeman, what he witnessed. Meanwhile, a blackout is announced and the bus stops to wait for its conclusion. Because of the blackout, the bus cannot be contacted by radio, so Eddie Sloan, an off-duty bus driver, offers to try and reach it in the dark. The saboteurs also realize that the bus will stop and so will not be in the proper place when the bomb explodes. They reach the bus before the others and hijack it. A chase ensues. Silva cuts the brakes and sets the bus rolling downhill toward the fields. Reba grabs the wheel and tries her best to guide the runaway bus. As they careen down the hill, the bomb breaks loose from the orange crate. In the meantime, Eddie and Quinn have captured the saboteurs and they hurry after the bus. When they arrive, they find that the passengers have managed to stop the bus and henpecked Henry has defused the bomb. Reba and Ryan celebrate with a kiss.