At the private island retreat of wealthy industrialist George Stark (Teodor Corrà), a group of people have assembled for a weekend getaway. Among the guests is Professor Gerry Farrell (William Berger), a scientist badly in need of a vacation. The first night passes uneventfully, but Farrell is enraged the next morning to discover that Stark and several of the guests planned the weekend to coerce him to sell his latest invention: a formula for industrial resin.
Farrell is immediately established as a decent man, and although his marriage to his wife, Trudy (Ira von Fürstenberg), appears to be content, she is actually involved in a secret romance with Stark's artist wife Jill (Edith Meloni). Stark's business partner, Nick (Maurice Poli), is verbally abusive to his coquettish wife Marie (Edwige Fenech), but does not object to her sleeping with other men, one of whom is the houseboy and manservant Charles (Mauro Bosco). Stark is less of a husband to Jill than he is a business manager. He arranges for her to have her paintings and artwork publicly displayed and is also a source of constant criticism. The sole happy couple appears to be Nick's co-worker Jack (Renato Rossini) and his wife Peggy (Helene Ronee). Also among the guests is Isabelle (Justine Gall), a teenage girl who is in Stark's care while her parents are away.
As Stark, Nick, and Jack badger Farrell for the secret of the formula by offering him $1 million each from their Swiss bank accounts, Jill discovers the dead body of Charles on the beach. Having already sent the motor launch away to prevent Farrell from leaving the island, and with the radio out of commission, Stark has no way of contacting the mainland. So, Charles body is moved into a large walk-in freezer where it hangs with other 'chunks of meat'.
The next morning, Farrell himself is the next victim. While he is walking alone on the beach, a sniper shoots him down. Trudy and Jill, walking hand-in-hand nearby for some alone time, hear the gunshot and find Farrell's body and run away to tell the others. The sniper is revealed to be Isabelle, who drags Farrell's body to the sea.
As tempers flare to Farrell's killing and "disappearance", the killings then escalate. Peggy, standing on the balcony of her room, is shot to death by an unseen assailant. Jack arrives on the scene first and accuses Stark of being responsible, which he denies. Marie turns up dead, having been tied to a tree with a knife sticking out of her chest. Jill turns up dead in her bathtub after being electrocuted. Each of the bodies is placed in the freezer. Rather than risk any more bloodshed, the four remaining survivors of Stark, Jack, Nick, and Trudy hold up in Stark's living room for the night. In this way they believe, the killer cannot strike without revealing him or herself. After bickering with Trudy, Nick storms off into the night. The next morning, he too is found dead, and his body is placed in cold storage.
Up to this point, Stark is the most probable suspect when he sneaks out of the house and uncovers a motorboat which will take him to the mainland. As he returns to the house to get supplies, Jack confronts him and reveals himself to be the killer, having killed off everyone to steal the $1 million cashier's checks from their bank accounts. Before Stark can do anything, Jack shoots him dead. But it is revealed that Jack has not been acting alone. After killing Stark, Jack meets in the freezer with Trudy, who is the real mastermind behind the whole scheme. Trudy had met with Jack sometime before arriving on the island and offered to kill everybody so she can steal the million dollar cashier checks from Stark, Nick, in order to hand him over the formula for the resin. Jack was even forced to kill Peggy because she got too close to discovering his plan. Jack then offers to hand Trudy over the three $1 million cashier checks, as she produces the microfilm for the resin formula. But Trudy tries to pull the same trick on Jack, by shooting him in the head, and stealing the checks for herself. But having anticipated Trudy's betrayal, Jack manages to shoot her with his gun before expiring. At this point, Isabelle, who has been hiding in the secret passageways in the house throughout the film, enters the freezer where Trudy and Jack killed each other, and calmly collects the three checks from Trudy's handbag.
Several months later, a well-dressed Isabelle arrives at the state prison to visit a very much alive Farrell who is incarcerated there. It turns out that Farrell did not develop the resin, but stole it from the real inventor after murdering him. He concocted the entire murder-for-money scheme with Trudy; Jack was an unwitting stooge. To conceal his involvement, Farrell convinced Isabelle that the others were trying to kill him, and had her drug him to make him appear dead. Isabelle pushed him out to sea, hoping someone would rescue him. His rescuers were the police on their way to Stark's island; the drug Isabelle used made him so delirious that he confessed the entire scheme. Isabelle gloats over her possession of the $3 million, and leaves Farrell behind to be hanged for his crimes.William Berger: Professor Fritz Farrel
Ira von Furstenberg: Trudy Farrel
Edwige Fenech: Marie Chaney
Renato Rossini: Jack Davidson
Helena Ronee: Peggy Davidson
Teodoro Corrà: George Stark
Justine Gall: Isabel
Maurice Poli: Nick Chaney
Five Dolls for an August Moon was one of Bava's most obscure films, and did not receive an official American release until 2001 when Image Entertainment distributed it on DVD. After the Image disc went out of print, Anchor Bay re-released as part of the "Mario Bava Collection Volume 2" box set on 23 October 2007. The film was released in France as L'île de l'épouvante / Island of Terror. In 2013, Kino International released the film on Blu-ray in the United States.
AllMovie gave it two stars out of five, calling it "a confusing and not terribly exciting whodunit."
In his review of the 2013 American Blu-ray release for Slant Magazine, Budd Wilkins writes "Five Dolls for an August Moon isn't top-tier Bava by any means, but for those with eyes to see, there are pleasures aplenty to be gleaned from this playfully abstract jeu d'esprit."
In his commentary track for the Kino International Blu-ray release of the film, Mario Bava scholar Tim Lucas argues strongly against Bava's own assessment of the film as one of his worst and calls it one of his most beautiful and innovative films.