After nightclub waiter Pierre is forced into a car by a gun-toting gangster named Trigger, bumbling busboy Myron Mertz tells his longtime friend, Larry Todd, the club's singer, that Trigger's boss Shorty demanded that Pierre be brought to him because Pierre had been romancing Shorty's girl friend Rosie. As Larry also has been flirting with Rosie, he departs in a panic. When Shorty then telephones the club, asking for Larry, Myron decides to help his friend by going to the gangster's hotel room and pleading his case. At the hotel, meanwhile, Mary Carroll, who occupies the room across from Shorty's, arouses Trigger's suspicions when a thunderstorm causes the lights to go out and she jokingly comments that it is a good night for a murder. In her suite, Mary then discusses the castle she recently inherited from her grandfather with the estate's executor, Cortega. Mary looks forward to seeing the castle, which is on Lost Island, off the coast of Cuba, but Cortega warns her that it has long been haunted. After Cortega reveals that an anonymous buyer has offered $50,000 for the castle, Mary receives a call from a stranger named Ramon Carriso, who advises her against selling the castle and asks to meet her. Myron, meanwhile, arrives at the hotel, but loses his nerve outside Shorty's suite. Myron allows his conscience to talk him into continuing and soon is being roughed up by Shorty's thugs, who have murdered Pierre. Just then, Larry shows up and sees Ramon lurking in the hallway. Seconds later, Ramon and Cortega, who is in his room, shoot at each other, and a confused Larry draws his own gun and fires it in Ramon's direction. The shots cause Shorty and his men to flee, and Myron dashes into the hallway, while Cortega surreptitiously grabs Ramon's gun. Hearing the police arrive, Larry forces his way into Mary's suite and begs her to protect him. Larry assumes the victim was one of Shorty's men and tells Mary that he shot in self-defense. When the police knock on her door, Mary shoves Larry into her bedroom and tries to hide Larry's dropped polka-dotted scarf, which another guest has identified as belonging to the gunman. After the police search the bedroom and leave empty-handed, Mary and Myron realize that Larry shut himself in her trunk, which has since been sent to the pier from which Mary's ship to Cuba is sailing. Myron rushes to the pier and frees Larry from Mary's trunk, but when Myron sees Trigger searching the pier, he jumps into the trunk. A drunk then comes along and, hearing Myron conversing with Larry, assumes that Larry is an amazing ventriloquist and tells a passing policeman about him. When Mary arrives, Larry abandons the still-encased Myron to speak with her, and Myron ends up on the ship. With Trigger and the police still in the vicinity, Larry decides to board the ocean liner with Mary. The ship sails before Myron is released from the trunk, but he quickly arranges transportation back to shore. Larry, however, has read a newspaper story about the shooting, which identifies Ramon as the victim and states that he was killed with a .38 caliber gun, not a .32 caliber like Larry's, and realizes that he did not fire the fatal bullet. Aware that Mary was to meet Ramon and that she has received a threatening note, Larry declares to Myron that they are going to Cuba to protect her. During the voyage, Larry and Myron join singer Carmelita Castina in entertaining the passengers, and Larry saves Mary from a falling fire bucket that has been dropped by a mysterious stranger with a scarred arm. Later, while Cortega beseeches Myron and Larry to keep Mary away from the island, Mary finds a knife stuck in her cabin door. She then runs into Tony Warren, an old friend who lives in Cuba, and he identifies the knife as a voodoo artifact and talks about the zombies on Lost Island. Upon landing in Cuba, Larry and Myron slip away from their hotel and row to Lost Island. As they enter the castle, they are spotted by an old woman, who orders her zombie son to follow them. Inside, Larry and Myron see ghostly apparitions and a portrait of Mary's great-great grandmother, whom Mary closely resembles. Mary then arrives in another boat and hears a disembodied voice warning her to leave. Myron and Larry are stalked by the zombie, but when Mary suddenly appears in her ancestor's dress, the zombie becomes confused long enough for Myron and Larry to lock him in a closet. While Myron keeps watch upstairs, Mary and Larry inspect the castle's downstairs mausoleum and soon discover clues to a hidden treasure. When Mary and Larry play certain notes on the mausoleum's organ, one of the coffin lids opens, revealing a fatally wounded Cortega. With his dying breath, Cortega tells Mary that he tried to protect her from murderous treasure hunters. The spooked Myron then falls into the mausoleum through a trap door, and Mary and Larry uncover the entrance to the underground treasure room. After descending, Larry and Mary are surprised by Ramon's twin brother Francisco, who demands to know who killed Ramon. Just then, Tony, whose arm is scarred, appears and shoots at Francisco, wounding him. Tony admits that after he unearthed the treasure, he hired Cortega and Ramon to scare Mary away, but killed Cortega when he turned against him. As Tony is about to kill Mary and Larry, Myron starts to play "Chopsticks" on the organ and triggers another trap door to open under Tony. Later, with Tony out of the way, Larry and Mary plan their Lost Island wedding. When they see skeletons sporting the heads of Bob Hope and Bing Crosby, however, they run from the castle in terror.