After her husband Kurt dies in a plane accident, Lisa Baumer receives one million dollars, which she is to collect in Athens, Greece. Lisa's ex, Philippe, stalks her and demands a payoff. When they meet to meet, she finds Philippe dead and flees to catch her flight to Greece.

Insurance investigator Peter Lynch is hired to tail Lisa to affirm the legitimacy of the payout. He meets Lisa when his cover is blown at the hotel. Also following Lisa is Kurt's mistress, Lara Florakis, along with her muscle, Abad Sharif. Cornering her at an abandoned theater, they try to get the money, but Lisa runs away. They try to capture her, but Lynch helps Lisa escape. Once Lisa gets the money, she arranges to meet her lover in Tokyo. However, a masked killer breaks into Lisa's hotel room and slashes her to death, before stealing all the money.

The police of Athens are on the case, led by Inspector Stavros. Also investigating is Interpol agent John Stanley, who had been following Lisa since the airplane explosion, which his agency believed was a bombing. Lynch and Florakis are questioned. Soon after, a journalist, Cléo Dupont, meets Lynch. They soon start a relationship and agree to investigate together.

Lynch himself goes to question Florakis, where Stanley also arrives. Sharif narrowly kills Lynch with a hatchet. Florakis plays dumb when Stanley also questions her, but Lynch accuses her and Sharif of being the killers. That night, the killer murders her. Sharif, arriving from her distress call, chases the killer to the rooftop, where he falls to his death.

The Lisa's lover, George Barnet, reconvenes in a hotel with a stewardess, his girlfriend. Barnet also has a uniform in his possession. One night, when Lynch joins Cléo at her studio apartment, the killer attacks her. Lynch arrives and saves her. At the scene is a cuff link, in the shape of a scorpion. An image of Kurt shows him with the same cuff links, and Lynch surmises Kurt is alive and in turn the killer. Meanwhile, Barnet is attacked and killed in London.

Lynch and Cléo arrange a trip on Lynch's yacht as part of a getaway. In the meantime, the police meet the stewardess, noticing her lapel pin is the same design as the cuff link. While on the yacht, Cléo sees Lynch dive into an underwater cave. When Cléo reaches air, she finds in the rocks of the cave floor a plastic bag with the insurance money. Lynch catches her and forces her back to the boat, admitting he really was the killer all along. Lisa's lover was his accomplice, and the bomber of the plane, and the whole plot was arranged to lure Lisa where she was to be killed, all to take the money. Lynch escalated to killing all liabilities, including Barnet, who was arranged to attack Cléo to divert suspicion from Lynch.

When Lynch offers to escape with Cléo, she rushes to the radio and tries to call for help. Lynch subsequently chases after her. Cléo stabs Lynch's shoulder with a harpoon, dives into the sea, and flees to a nearby island, Lynch in tow not far behind. When he catches up to her, Lynch is shot by the police.

• George Hilton as Peter Lynch
• Anita Strindberg as Cléo Dupont
• Alberto de Mendoza as John Stanley
• Ida Galli as Lisa Baumer
• Janine Reynaud as Lara Florakis
• Luigi Pistilli as Inspector Stavros
• Tom Felleghy as Mr. Brenton
• Luis Barboo as Sharif
• Lisa Leonardi as Hostess
• Tomás Picó as George Barnet

AllMovie called it a "devilishly entertaining giallo thriller".^[4]

Adrian Luther Smith wrote "....one of the best of Sergio Martino's impressive batch of thrillers. It has considerable energy, some pleasing exotic locations and is crammed with a great cast of shady characters.....The resulting gory carnage is graphically presented."^[5]

• The Case of the Scorpion's Tail at IMDb