A hiker finds a dead woman on the Yorkshire moors; the victim, who has been strangled, is identified as Alice Ruber. Shortly afterwards, Belgian detective Hercule Poirot (Peter Ustinov) examines a diamond belonging to millionaire industrialist Sir Horace Blatt (Colin Blakely), and declares it a fake. Assured that Sir Horace gave a real diamond to his mistress and she later returned the fake to him, Poirot accompanies Sir Horace to confront her at an exclusive island resort. The hotel is the former summer palace of the reigning King of Tyrania, now owned by Daphne Castle (Maggie Smith), who had received the palace "for services rendered".
Sir Horace's former mistress is actress Arlena Stuart Marshall (Diana Rigg), who is on holiday with her husband Kenneth (Denis Quilley). Arlena is emotionally abusive to her stepdaughter, Linda (Emily Hone), and flirts with handsome young Patrick Redfern (Nicholas Clay), who is himself married to the meek Christine (Jane Birkin). Patrick is on the island only because Arlena arranged it. Kenneth turns to his old friend, Daphne, who reviles the way that Arlena treats both him and Linda. Arlena has also caused financial problems for Odell and Myra Gardener (James Mason and Sylvia Miles) by walking out of a major play, and refusing another. Writer Rex Brewster (Roddy McDowall) has already spent the royalties advanced to him for a tell-all biography of Arlena. Early one morning, Arlena takes a paddle-boat to Ladder Bay. Patrick and Myra go for a boat trip around the island and see a body lying motionless on the beach. Patrick approaches the body and recognizes Arlena, announcing that she has been strangled. Poirot must determine which of his eight fellow guests—or Daphne—is the murderer.
Daphne had heard Kenneth in his room typing at the time of the murder, and Christine was with Linda at Gull Cove and did not leave until 11:55 for a 12:30 tennis match. Sir Horace argued with Arlena about the diamond at Ladder Bay at 11:30, which is confirmed by his yacht crew and by Daphne. Arlena kept the diamond, promising an explanation that evening, and Poirot finds the jewel nearby in a grotto. Patrick left at 11:30 with Myra, seeing Sir Horace's yacht coming, and hearing the noon cannon. Rex met Linda entering Gull Cove at 12:00, and reports that a bottle flung from the top of a cliff nearly hit him. Odell was seen reading by Daphne and her staff. He claims low water pressure hindered his 12:15 wash before tennis, but nobody admits to bathing at that time.
Assembling the suspects together, Poirot accuses Christine and Patrick of the crime: Christine knocked out Arlena and hid her in the nearby grotto, and Patrick strangled the helpless Arlena later. Christine posed as Arlena with self-tanning lotion, Arlena's swimsuit and large red hat, to be mis-identified by Patrick. But Poirot had smelled Arlena's perfume in the grotto. Christine set Linda's watch twenty minutes fast, suggested a swim cap to muffle the noon cannon, and corrected the watch afterward. She tossed out the lotion bottle, almost hitting Rex, and bathed off her tan, thus depriving the leaky hotel water system of pressure. Poirot suspects that Patrick switched Sir Horace's jewel with a paste one, and that Patrick and Christine killed Arlena to protect the theft. The Redferns scoff at the detective's accusations, as he has no real proof.
On leaving the hotel, Patrick pays by cheque, signing the "R" in "Redfern" in a distinctive way that Poirot recognizes as the same way "Felix Ruber", husband of the Yorkshire moor victim, signed his name. The hiker that found the body had been Christine, establishing Patrick's alibi. Poirot knows photos from the British police will show Patrick and Felix to be the same person. Patrick puts a pipe in his mouth that has never been lit during his stay; Poirot empties the pipe bowl to reveal the diamond.Peter Ustinov as Hercule Poirot, the famous Belgian detective
Maggie Smith as Daphne Castle, the owner of the island resort
Colin Blakely as Sir Horace Blatt, a millionaire industrialist
Diana Rigg as glamorous actress Arlena Stuart Marshall
Denis Quilley as Kenneth Marshall, Arlena's husband
Emily Hone as Linda Marshall, Kenneth's teenage daughter from an earlier marriage
Roddy McDowall as Rex Brewster, a pompous writer and theatre critic
James Mason and Sylvia Miles as Odell and Myra Gardener, New York City theatrical producers
Nicholas Clay and Jane Birkin as Patrick and Christine Redfern, a handsome young man and his mousy wife
The screenplay was written by Anthony Shaffer (who had worked on previous Christie adaptations) and an uncredited Barry Sandler. The adaptation stayed fairly close to Christie's work but truncated scenes for time constraints, removed minor characters, and added humorous elements that were not present in the novel. Additionally, the novel was set in Devon, but the film was set on an Adriatic island in the fictional kingdom of Tyrania (based on Albania). The characters of Rosamund Darnley and Mrs. Castle were merged, the characters of Major Barry and Reverend Stephen Lane were omitted, and the female character of Emily Brewster was written as a man named Rex Brewster, played by McDowall.
Ustinov made his second film appearance as Poirot, having previously played the Belgian detective in Death on the Nile (1978); Smith and Birkin also appeared in both films. Quilley and Blakely appeared in the earlier Brabourne-produced Murder on the Orient Express (1974). Guy Hamilton had previously directed another Agatha Christie film, The Mirror Crack'd, in 1980.
Costumes for the film were designed by Academy Award winner Anthony Powell.
The film was shot at Lee International Studios in Wembley, London, and on location in Majorca, Spain.
The actual island used for aerial shots is Sa Dragonera, an uninhabited islet with "natural park" status, located just off the west coast of Majorca near Sant Elm. Other locations used were Cala Blanca as Ladder Bay, and offshore at Sant Elm for the south of France (Sir Horace's boat scenes). Cala d'en Monjo was used for the exteriors of Daphne's Cove and Hotel; the hotel itself was a private estate later bought by the Island Council of Majorca (along with Ajuntament de Calvià - Municipal) to create a natural park, which was demolished to its foundations. Gull Cove is the remote Cala en Feliu on the Formentor Peninsula. The other hotel exterior shots were filmed at the Raixa Estate, a large Italian style villa surrounded by Pietro Lazzarini-designed gardens, located in Bunyola, north of Palma. Once owned by the German designer Jil Sander, it was subsequently purchased by the Island Council of Majorca. Finally, Poirot boards his boat to the island from Cala de Deià, the coastal area of the exclusive town.
The early scenes on the moors were shot in the Yorkshire Dales, England, with the exterior of the police station being the former Literary Institute in Muker, Swaledale.
Evil Under the Sun was Peter Ustinov's second portrayal of Christie's Belgian detective Hercule Poirot. Ustinov played Poirot in a total of six films:Death on the Nile (1978) – theatrical film
Evil Under the Sun (1982) – theatrical film
Thirteen at Dinner (1985) – TV film
Dead Man's Folly (1986) – TV film
Murder in Three Acts (1986) – TV film
Appointment with Death (1988) – theatrical film