This movie is the second in A.I.P.'s H.G. Wells film cycle, which includes The Food of the Gods (1976) and Empire of the Ants (1977).
Ship's engineer Andrew Braddock (York) and two other men are floating in a lifeboat in the middle of the Pacific following the wreck of the ship Lady Vain. After seventeen days at sea, they land on an island where the two men accompanying Braddock are promptly killed by animals. Braddock is nursed back to health in the compound governed by the mysterious scientist "Dr. Moreau" (Lancaster). Besides Moreau, the inhabitants of the compound include Moreau's associate, Montgomery (Davenport), a mercenary; Moreau's mute, misshapen servant, M'Ling (Cravat); and a ravishing young woman named Maria (Carrera). Moreau warns Braddock not to leave the compound at night.
Moreau welcomes Braddock as an honored guest and willingly shares his fine library, but there are some strange goings-on. One day Braddock witnesses Moreau and Montgomery manhandling a chained creature who is clearly not quite human, and the island is home to more than just this one. Moreau explains that they are, in fact, the hybrid products of his experiments upon various species of wild animal. Braddock is both shocked and curious. Moreau explains that he is injecting the animals with a serum containing human genetic material. At times the human/animal hybrids still have their animal instincts and don't quite behave like a human which sometimes enrages Moreau, feeling that his experiments haven't worked successfully. That night, as Braddock is reeling from learning the truth, Maria goes to his room where they make love.
The following day, Braddock takes a rifle and leaves the compound, determined to see exactly how the hybrid creatures live. He enters a cave and finds several of them (all male). Just as he is surrounded by them and about to use the rifle to defend himself, Moreau appears and restores order. The Sayer of the Law (Basehart) is the only one of Moreau's experimental beasts who can speak; Moreau calls on him to utter the three laws (no going around on all fours, no eating of human flesh, no taking of other life) aloud to the other creatures. This reminds them that they must not attack Braddock.
After one of the man-beasts, (the Bull man) kills a tiger, Moreau intends to take it to the "house of pain", his laboratory, as punishment. The man-beast panics and runs. Braddock finds it in the jungle, badly injured, where it begs him to kill it rather than return it to the lab. Braddock shoots it, angering the man-beasts, as Braddock has broken the law of killing.
Convinced that Moreau is insane, Braddock prepares to leave the island with Maria. Moreau stops them and straps Braddock to the table in his lab. He then injects him with another serum so that he can hear Braddock describe the experience of becoming animalistic. Caged, Braddock struggles to maintain his humanity. When Montgomery objects to this treatment, Moreau shoots him in cold blood.
Outside the compound, the angry man-beasts turn on Moreau because by killing Montgomery he has broken the very rule he expected them to follow. He is killed at the compound's gate while trying to whip his attackers into submission. The man-beasts, now overpowered by their primitive natures, rampage through the compound. Braddock, Maria, and M'Ling engineer an escape, but the compound is burned and most of the man-beasts are killed by the wild animals which Moreau kept for his experiments. During the final escape, M'Ling risks his life to save his companions from a lion and both fall into pit trap.
Braddock and Maria manage to float away in the lifeboat that Braddock arrived in, but are followed by one of the last man-beasts. After a battle with each other, Braddock kills the man-beast with a broken oar. Some time later, they are rescued by a passing ship, and the serum has worn off, returning Braddock to his full human state.
The movie was filmed in Saint Croix, U.S. Virgin Islands.
In terms of casting, Lancaster has been described as perfectly matching Wells' description of Moreau's physical appearance, unlike the other two actors to play the role on screen, Charles Laughton in 1932's Island of Lost Souls and Marlon Brando in 1996's The Island of Dr. Moreau, both of whom were more portly and with receding hair.
Actress Barbara Carrera claims there were 3 or 4 different endings imagined, including one in which her character gave birth to a kitten. That version was favored by Producer John Temple-Smith, which actor Michael York flatly refused to do. Director Don Taylor said that he didn't take it seriously and the footage was never shot.
Upon its original release, the film received mixed reviews, though the performances, including those of Lancaster and York, were praised. It currently holds a 50% rating on review aggregate Rotten Tomatoes and a 5.9 on the Internet Movie Database.
The film premiered on 13 July 1977 in the United States. Lorber Films released the film under its Kino Lorber Studio Classics imprint available for the first time on Blu-ray in the U.S.