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Nelson Brenner is a double agent, employed by the CIA but working under cover for some unspecified enemy power. Brenner wants to get rid of fellow CIA agent 'Geronimo' to eliminate a threat to his double identity. Brenner adopts a fake identity in the form of Mr Steinmetz, a shadowy character who pretends to hire Geronimo for a job. Geronimo is lured into a meeting with Lawrence Melville, who believes he is working for Steinmetz, although he is unaware of his real identity. After meeting Melville, Brenner shows up and murders Geronimo by clubbing him to death with a tire iron and leaving the body on the beach under a pier, hoping to make the murder look like a mugging gone wrong.
Columbo is assigned to the case and uncovers a link between the murdered man and Brenner by means of photos taken at a fun fair where Brenner and Geronimo had met earlier that day. His investigation leads him into a confrontation with the CIA who are unaware of Brenner's double agent status and try to deter Columbo from investigating one of their top agents. Columbo, of course, is convinced of Brenner's guilt and eventually discovers the fatal flaw in his murder plan.