NINE GUESTS FOR A CRIME

Four men, on the seashores of a small island, kill Charlie, one half of a young couple who were having sex on the beach, by shooting him and then being buried alive under the sand.

Twenty years later, the rich Ubaldo, owner of a luxurious villa on the island where Charlie's crime took place, organizes a tourist excursion to the place. His three children are also with him: Lorenzo, Michele and Patrizia. The children are accompanied by their respective spouses: Greta, Carla and Walter.

With them is also Elisabetta, Ubaldo's sister and former lover of the deceased Charlie, as well as Giulia, Ubaldo's new young wife. Once at the villa, a mysterious assassin kills the sailors of the boat that had transported the nine to the island.

The group, therefore, remains trapped on the island. In the meantime, the assassin strikes again, killing the various guests of the villa one by one. At the end, only Elisabetta and Lorenzo, Ubaldo's eldest son, remain alive, but neither of them is the murderer.

The murderer, in fact, had staged her death immediately after landing on the island: she is Charlie's daughter who intends to avenge her father killed years ago by Ubaldo, Lorenzo, Michele and Walter. Surprisingly, it turns out that Charlie's daughter is Carla, Michele's wife. The assassin was able to count on the complicity of her mother, Elisabetta. Lorenzo admits he knew from her body being the only one not recovered. She faked her drowning, delighted to have seen the whole family squirm from that point onward while she avenged her dead father.

When the sun comes up, Lorenzo marches the women to the motorboat at gunpoint. Elisabetta demands to know why Lorenzo didn't stop Carla when he figured her out, Lorenzo confessing he wanted his family dead anyway. Demanding the spark plugs back, Carla says she'll take Lorenzo to the location. She retrieves them, but then runs, Elisabetta stopping Lorenzo from chasing her. Lorenzo shoots Elisabetta and breaks from her grasp to run after Carla. Lorenzo catches up to Carla and shoots her, taking the spark plugs from her hand. However, Elisabetta dumps gasoline into the boat before she too dies. When Lorenzo tries to start the propeller, the boat bursts into flames, immolating him instantaneously, as the wounded Carla watches from the rocks and dies.

The killer is revealed to be Carla, daughter of the murdered Charlie, with help from her mother Elisabetta. In a final confrontation, most of the remaining family members die, culminating in Lorenzo's attempt to escape, which ends in his death when the boat explodes. Carla, wounded, dies watching from the rocks.

The finale ties the cycle of vengeance back to the original crime, showing how old sins can consume an entire family. By making the avenger one of their own, the film underscores the corrosive effects of guilt and unacknowledged wrongdoing. The explosive climax leaves no one untouched, with retribution claiming victims on all sides, a fittingly bleak resolution for this sun-drenched giallo.