A boyhood friend of Admiral Nelson joins an insurance syndicate, falls in love with a married woman, and tries to preserve Nelson's fleet for England.
Young Jonathan Blake (Freddie Bartholomew) works in his mother's ale house and goes aboard a pirate ship with young Horatio Nelson (Douglas Scott); they learn of insurance fraud and escape by swimming. Horatio is punished at home and is made a midshipman. Jonathan goes to London to inform Lloyd's of the fraudulent ship sinking. John Julius Angerstein (Guy Standing) explains syndicates to Jonathan, who asks for a job. Angerstein teaches Jonathan to share information honestly.
In 1784 Lloyd's moves. Angerstein rejects Lord Everett Stacy (George Sanders) as a gambler. Jonathan (Tyrone Power) is accused of peeping and is fined fifty pounds. He tells Angerstein of a way to send messages across the channel. Jonathan tells Elizabeth Stacy (Madeleine Carroll) that Napoleon has ordered all the English in France arrested. Jonathan with a pistol helps her escape to England by sailing a boat. They dine, and Jonathan kisses her. Elizabeth left early, and he eventually finds her in a palace dancing. Jonathan learns her husband is Stacy and leaves. Jonathan drinks, and Polly (Virginia Field) tries to console him.
In 1803 lucky Jonathan runs a syndicate that insures anything; but Angerstein calls it gambling and cynicism. Jonathan bets he can introduce Polly to the Prince of Wales and wins. Jonathan dines with Elizabeth after her husband leaves. She explains and meets Jonathan at an art studio. Stacy asks Jonathan to let him invest 5,000 in his syndicate. Old Q (C. Aubrey Smith) wants more than friendship with Polly. A British fleet is destroyed and captured. Angerstein says all claims must be paid. Lloyd's raises rates, and shipping stops. Angerstein proposes military convoys; but Jonathan objects to Nelson's fleet being reduced and suggests lower rates. Stacy asks Jonathan for 1,000 pounds to stay out of debtor's prison. Elizabeth offers Stacy her money for a divorce. Jonathan tells Elizabeth that French ships escaped Nelson, and she offers him her fortune.
In 1805 bad news means half of Nelson's ships will be taken for convoys. Jonathan gets a letter from Nelson and sends a message of Nelson's victory so that convoys are canceled. Elizabeth tells Stacy she changed her mind, and Stacy tells Angerstein of Jonathan's fraud. Angerstein confirms it, and Jonathan goes home. Angerstein tells Stacy that his wife's fortune is in Jonathan's syndicate. Stacy does not denounce Jonathan but finds him with Elizabeth and shoots him. On a ship Nelson is wounded but still gives orders. Elizabeth urges Jonathan to live. Angerstein tells Jonathan of Nelson's victory at Trafalgar. Jonathan gets up to see Nelson's funeral carriage and remembers their boyhood pact.
This story explores the dilemma of insuring shipping during the war against Napoleon. Jonathan wavers between romance and cynicism; then he risks fraud to help his Navy friend protect England rather than its ships.