Captain Ben Cutlet is permitted to extend his stay at his sister's public house because of his success in drawing customers with his tall stories of heroic adventures on the seven seas which differ in details with every telling. The rotund young Albert and old-timer Jeremiah Harbottle help behind the bar.

Cutlet is befriended by Olivia Potter-Porter, the widow of a seafarer and co-owner of a shipping line, who fails to realise his ignorance of all maritime matters and asks him to address a meeting of the local Sea Scouts. There he gets into a tangle when Albert asks him how to tie a particular knot, the short sheepshank, and his incompetence is observed by Olivia's crooked partner, Yates, and first mate Jim Maryatt.

As the captain of the shipping line's vessel, the Rob Roy, has resigned rather than take it to sea in its present dangerous condition, Yates decides to have Cutlet appointed in his place, to take the blame after Maryatt scuttles the ship so that Yates can gain the insurance.

Albert and Harbottle find a photograph of Cutlet which reveals him to have been no more than captain of a canal barge, and he admits to them that he has never been to sea in his life. However, he accepts the post of captain after he has arranged for Harbottle to deliver a fake telegram about a sick relative just before the ship sails so that he can withdraw on compassionate grounds. When Harbottle fails to appear, Cutlet is forced to set sail, taking part of the pier with him, and is soon seasick. He finds Harbottle and Albert have stowed away in his cabin. Maryatt catches the pair eating his breakfast and puts them to work.

After the ship has sailed around Cape Horn into the Pacific (rather than towards Norway, as Cutlet had been led to expect), Maryatt and the men take control, tying up the Captain and Harbottle. Albert is spared when he pretends to side with the mutineers but soon frees the pair and escapes with them on a raft, taking a battery-operated radio receiver which the ravenous Cutlet throws overboard when a cookery show gets on his nerves.

They wash up on the shore of a tropical island and are surrounded by cannibals. The radio drifts in after them, still noisily blaring away. The broadcast of planes flying over at an air show scares the islanders and Cutlet pretends to worship the set. He persuades the islanders to respect the "voice in box" and settles down to a comfortable existence with Albert and Harbottle. Maryatt and the others arrive in a rowboat some time later and are taken captive.

Then the Rob Roy appears, still afloat, and Cutlet sails for home with the mutineers imprisoned in the hold, helped by Albert, Harbottle, and the ship's engineer who had prevented it from sinking. The Rob Roy hits a millionaire's yacht stranded on a sandbank, dislodging it. Cutlet is acclaimed for his fine seamanship but his hero's welcome as he returns to port is spoilt when the ship knocks off the end of the jetty.