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On Key West, Florida, former naval officer Sam Martin ekes out a living chartering his cabin cruiser to tourists.  One morning Sam and his first mate Harvey are stopped by Cuban revolutionary sympathizers Carlos Contreras and his partner Juan, who ask to hire Sam’s boat.  Wary of becoming involved with the nearby political unrest, Sam refuses.  Sam and Harvey then take tourist Mr.  Peterson out fishing, and he loses the expensive tackle at sea.

Later, Peterson grudgingly promises to pay Sam his near thousand-dollar, ten-day debt in full that evening.  At Freddy’s bar, Sam assures the easy-going bartender he will finally pay his long-overdue bill.  Gas station owner Arnold finds Sam at the bar and demands payment for three months of gas for the boat and agrees to meet Sam at Peterson’s hotel that night.  Sam then stops at home to see his wife Lucy and she reminds him about back payments on the boat to lender Sy Phillips.  That evening, when Sam meets Harvey and Arnold at the hotel, he sees Peterson being taken to jail for passing bad checks.

Dismayed, Sam visits Pop’s pawnshop, where he hocks a clock for twenty dollars, then takes the money to the small illegal gambling parlor behind Pop’s shop.  Sy attempts to talk Sam out of gambling, but Sam refuses and promptly loses.  Nearby, jovial gambler “Papa” Hanagan, overhears Sy and Sam discussing the money owed on the boat.  The next morning, Hanagan and his Swedish girl friend Eva meet Sam on the pier and hire him for a day trip.

Once at sea, Hanagan asks Sam to stop in Havana, but Sam declines, explaining that the ongoing revolution makes Cuba legally off-limits.  Nevertheless, Hanagan pressures Sam to drop them off on the Cuban coast, just outside Havana.  Sam demands a large extra sum for the arrangement and is surprised when Hanagan agrees.  That night, when Hanagan and Eva miss their scheduled pick-up time, Sam anxiously slips into Havana to look for them and finds Eva at a bar.  Sam insists on being paid and Eva asks him why he is not curious about where Hanagan gets his money.  Sam agrees to return to the boat and wait for the couple.

Meanwhile, in a small room above the club, Hanagan and several Cuban revolutionaries negotiate a gun shipment promised by Hanagan.  On the way back to the boat, Hanagan kills a policeman who refuses to be bribed, then kills the cab driver once they arrive at the boat.  Sam is suspicious of Hanagan’s activities, and at sea carefully avoids the Coast Guard patrol boat.  Back at Key West, Sam pays Freddy and Arnold before being summoned by Commander Walsh, the head of the Coast Guard, who has already questioned Harvey.  Sam denies having gone to Cuba, then is stunned to hear of the two murders.  At home, Sam tells Lucy the prior night’s activities and she assures him that he is not responsible for the killings.

Later, Eva visits Sam to rehire him, but he refuses.  Eva reveals that Hanagan has purchased the boat from Sy and Sam is outraged.  When a policeman stops by with Hanagan’s ownership papers and the request for the boat’s keys, Sam frantically goes to Pop to ask for two thousand dollars to buy back the boat, but Pop regretfully refuses.  Sam worries about having to deal with Hanagan, but Lucy agrees that it is the only way to get back the boat.  The next day, Hanagan tells Sam that he will receive the boat plus a large bonus for a three-day trip to the other side of Key West and a Cuban coastal spot.  Sam returns home to tell Lucy of the plan, but rouses Harvey’s suspicions when he attempts to send him away for equipment.  Harvey secretly stows away on the boat, where Sam later hides a machine gun in the hold before Hanagan and Carlos arrive.

On the far side of the island, the group picks up two more men, including Hanagan’s anxious partner Buzurki, with two large boxes of rifles.  Carlos alone knows their destination of Ibarra, Cuba and gives Sam the route.  Later Carlos goes down to inspect the rifles and is furious to find the boxes full of rocks.  When he protests, Buzurki shoots him and Hanagan orders Sam to continue to Ibarra.  Harvey appears from the hatch at that moment in an attempt to save Sam, but is quickly overpowered.  When Hanagan wants to shoot Harvey, Sam points out that if Hanagan kills Harvey he must also kill Sam, who is the only one who can pilot the boat.  Hanagan then forces Harvey back into the hatch and the journey continues.

Near Ibarra, Sam fakes engine trouble, but when he opens the engine hatch discovers that Hanagan has already found the machine gun.  Harvey then slips overboard and swims toward shore.  Buzurki and the other man shoot at Harvey, but Sam guns the engines, knocking everyone off balance, and giving him the opportunity to snatch Hanagan’s gun.  Sam shoots all of the men, but is wounded by Hanagan before killing him.  Harvey returns to the boat to tend to Sam’s wound and pilot the boat back to Key West where Lucy is waiting.