Captain Jim Duncan meets with sailor Nick Shaw in a tavern and hires him to work as the first mate on the Island Princess , a freighter bound for Lisbon with a cargo of flour. At the end of the meeting, sailor Joe Miles summons Duncan to talk with a passenger named Thomas Nagle. The next day, Nick arrives at the ship to find that Duncan has been killed. Counterfeit bills are found on his body, causing the police to speculate that Duncan was planning to distribute the fake money in Europe. Nick is convinced of Duncan's innocence, however, and vows to clear his name. After Captain Stanton is hired to take Duncan's place, Nick checks in the passengers. He is especially interested in Nagle, because he met with Duncan the previous night. While he is searching Nagle's room, Nick is interrupted by attractive Norma Harrison, who claims to be Nagle's private secretary and nurse. That night, Nick searches the hold and discovers a crate filled with guns. Before he can search further, he is attacked and knocked unconscious. Butch, the passenger who attacked Nick, then reports to Nagle, and the two men return to the hold. Nick sneaks out and asks some of the crew members to help him move the guns, but by the time they return, Nagle and Butch have moved the guns to Nagle's cabin. Realizing that Nick presents a danger to his plans, Nagle invites him to take part in a poker game the next evening. Nagle lets Nick win and then pays him with counterfeit bills, confirming Nick's suspicions. Unknown to Nick, Nagle and the other passengers, who are really members of Nagle's gang, are planning a takeover of the ship. Nagle is trying to win Nick over to his side because he needs the first mate's navigational skills. After alerting his men, Nagle orders Stanton to change course for Marseille. When Nick learns about the change in course, he alerts the crew that the ship has been hijacked. The crew rallies for a counterattack, but Nick stops the conflict and sends the crew to their quarters. He then pretends to switch his allegiance to Nagle, but later, helps the crew regain control of the ship. Nagle, Norma and the others barricade themselves in the engine room with the ship's food supplies. Nagle orders his men to stop the engines, and then announces that they will stay there until the crew surrenders. Because Nagle's men smashed the radio, the crew is unable to send for help. While the radio engineer fixes the radio, Nick sets the other sailors to rigging up a set of sails. Using the sails, the ship heads for Lisbon. After the radio is repaired, Nick alerts the Lisbon police, who board the ship and round up the crooks.