The film starts with two fisherman, John Gurney (George Coulouris) and his son Hamp (Jack Watson), braving the fog to land on Snape Island. They find a group of butchered teens and one survivor, Penelope (Candace Glendenning), who is mad with fear and who kills Gurney.
She is hospitalised in a catatonic state and part of the film is the doctors trying to unlock the secret her shutdown mind holds.
Meanwhile one of the kids was murdered with a Phoenician spear made of solid gold and a museum sends a team to the island as they believe there must be caves holding a Phoenician burial chamber from 3000 years before – probably dedicated to the fertility god Baal. Hamp and a relative Brom (Gary Hamilton) take them out along with Evan Brent (Bryant Haliday) a PI hired by Penelope’s family to prove she didn’t murder her friends. However they are not alone on the island and Hamp, Brom and Brent each know more than they are letting on.
The imdb page is here.