"Commodore" Otis Swanson owns a boat building business that is being run by his son-in-law Charles Clay, who is married to his alcoholic daughter Joanna. Swanson is angry at the way Clay is handling the business and threatens to sell it, effectively disinheriting Charles and Joanna.
In this unconventional episode of Columbo, we do not see the murder being committed at the start of the show. Instead, we see Clay tidying up Swanson's private office in the yacht club, with Swanson's body on the floor. Clay then takes Swanson's body out to his (Swanson's) yawl at night and dresses up in Swanson's uniform to fool the coast guard station into thinking Swanson is taking his boat out for a late-night sail, as he often did. Clay takes the yawl a short distance out to sea, throws Swanson's body overboard and abandons the boat, using scuba gear to return to shore undetected.
Columbo and his young sidekick Theodore "Mac" Albinsky soon unearth clues that lead them to suspect Clay of murdering Swanson. However, their theories are derailed when Clay himself turns up murdered the next day. This episode thus turns into a genuine whodunnit rather than the usual "how is Columbo going to prove he did it", in that the real murderer is not revealed until the very end of the show.