[citation needed] On the remote Petrie's Island off the east coast of Ireland farmer Ian Bellows goes missing and his wife contacts the local police.

Constable John Harris finds the farmer dead in a cave without a single bone in his body and fetches the island's physician, Dr Reginald Landers, who is unable to determine what happened.

West and Stanley learn that a group of oncology researchers led by Dr Lawrence Phillips have a secluded castle laboratory on the island.

Thinking the doctors are at the castle, Constable Harris bikes up there to tell them about the discovery of a dead, boneless horse, only to wander into the laboratory's "test animals" room and be killed by an offscreen tentacled creature, the result of Dr Phillips's experiments.

After learning all they can from the late Dr Phillips's notes, West and Stanley recruit the islanders, led by Roger Campbell, to attack the silicates with everything they've got.

The story ends with evacuation and medical teams inbound from the mainland, and West commenting on how fortunate they were that this outbreak was confined to an island.

[6] The Monthly Film Bulletin wrote: "Although the plot is little more than a rehash of Day of the Triffids, it is quite effectively concocted and features some sluggishly mobile jellied turtles, each waving a snake-like tentacle ...

"[8] AllMovie's Brian J. Dillard wrote: .This creepy yet clunky sci-fi-horror flick boasts one of the coolest monsters ever to grace the silver screen—radioactive silicone beings ("silicates") that suck the calcium right out of your bones.

...With the exception of these creatures du jour and the eerie electronic sounds that emanate from them, Island of Terror is a fairly standard-issue lab-coats-versus-creatures flick in the mold of superior genre fare such as 1954's Them.