Shortly after joining the board of management of the Trust, Miss Marple witnesses the sudden death of fellow trustee Ffolly-Hardwicke, who has just returned from a surprise visit to the ship, much disturbed by something he has discovered there.

Resolving to learn what the murdered trustee had discovered, she visits the ship, while her dear friend and confidante, Mr. Jim Stringer investigates on shore.

Lt. Compton – who incites the boys aboard the Battledore to the robberies – overhears their conversation and is heading down, presumably to kill Miss Marple, when he is murdered – run through with a sword and then hanged from a mast.

She pretended to be in love with Sub-Lieutenant Humbert, whose family belongs to the upper-class, to receive invitations to social events and therefore to explore potential robbery targets.

Then, she announces to the crew that she knows that the poison was administered using a mousetrap as a booby-trap, and she hints that she intends to reveal the murderer's identity shortly.

Bacon secretly remain on board, hiding in wait for the murderer to reveal himself by attempting to silence Miss Marple.

In fact, he stayed behind to confess to Miss Marple his hidden seasickness, which she already suspected, since she observed him getting the respective medication from the sickbay.

After Dimchurch gratefully thanks Miss Marple he is about to leave the ship to belatedly join the celebrations onshore but is struck unconscious by an unknown man.

Miss Marple searches the ship for the loaded mousetrap, cautiously using a sword, not her hands, to poke into possible hiding places.

Breeze-Connington succeeds in disarming her and is about to administer the coup de grace, but Mr. Stringer, whom Miss Marple had thought was ashore, clubs him over the head from behind with a belaying pin.

As he enters the state-room to hear the verdict, he sees his sword on the table with the hilt toward him, and mistakenly infers that he has been found guilty.

He makes his farewell and turns to go, but Miss Marple stops him, saying, "I think I speak for my fellow trustees when I say that golden rule is hereby rescinded.