A man is pursued through a jungle by three others who take potshots at him, winging his arm. Frantically he searches for shelter and rushes into the nearest cave where he hides out while his attackers follow and end up down a hole that happens to contain a horde of gold bars. The men are delighted, realising they are rich, but then their quarry drops the rope ladder, trapping them as he gloats. They're not beaten however, and one manages to climb out only narrowly escaping being blown to bits by dynamite that kills everyone except him and sets off the volcano. Making it back to his ship, he tells his captain, Skinner (Terence Stamp) about his discovery... they will return.
Supposedly the most expensive Spanish film to that time, poor old Mystery on Monster Island has garnered little praise over the years, mainly because of some dubious choices by its producer and director Juan Piquer Simón, who also co-wrote the script, an adaptation of an obscure Jules Verne story (but not Mysterious Island, confusingly). Most of the cash seems to have been spent on getting the cast and crew out to exotic locations in the Canary Islands and Puerto Rico, as when the monsters of the title turn up, they're somewhat underwhelming.
Also less than impressive is the screen time given to the film's ostensible stars, Peter Cushing and Terence Stamp, as they appear for about five minutes at the beginning and five minutes at the end, and although they seem to be sharing scenes never actually share the screen in the same shot, so they could have been filmed weeks apart for all we know. Whether they met or not, doesn't really matter if you're willing to overlook their lack of contribution because they are chiefly present to set the plot in motion, which is that Cushing's movie son, Jeff Morgan (the considerably less famous Ian Serra) wishes to go off on an adventure before taking over the family business.
Although his father is reluctant, he allows it and soon Jeff has waved goodbye to his fiancée Meg (Ana Obregón) and set off with her elocution teacher Professor Thomas Artelect (David Hatton) - something not quite right there, shouldn't it be the other way around? Anyway, they set sail on the meaningfully named ship Dream and halfway through the voyage they're horrified to find the crew dead and sea monster men chasing them around - abandon ship! They wind up on the island we saw at the start of the film, apparently shipwrecked.
It's not long before they have settled in, however, after luckily finding the encampment of a previous expedition who are now apparently dead and before you can say Swiss Family Robinson the duo have a home of their own. This in spite of the antics of the highly strung Professor, a bungler who jumps at his own shadow and craves the comfort of civilisation that making friends with a chimpanzee doesn't provide. Hatton is incredibly broad in his comic relief performance, but in his defence I'd probably be reacting the same way if I had been washed up on an inhospitable island. In due course the beasts turn up, shuffling arthritically but of impressive size, as do a tribe of African-looking cannibals (in the Pacific?!). It's easy to look down on Mystery on Monster Island for its inanity, but it will remind the less critical viewer of the Amicus fantasies of the seventies like At The Earth's Core or The Land that Time Forgot and in that vein is amusing if undemanding. Catchy music by Alfonso Agulla, Alejandro Monroy and Carlos Villa.