Director Charles Marquis Warren was mainly known for his westerns, which he directed a very large number of. In 1955, he’d directed many of the early episodes of the popular and long-running western television series Gunsmoke (1955-1975) and would later produce the equally successful Rawhide (1959-1965) and The Virginian (1962-1971). Horror rarely featured in his extensive filmography and indeed it wasn’t until he shot The Unknown Terror back-to-back with its double bill mate Back from the Dead in 1957 for Twentieth Century-Fox’s B-movie unit, Regal Pictures, headed by Robert L. Lippert, that he made his only forays into the genre. And based on this duo, one has to assume that he was reasonably unfamiliar with the genre and a much better director of horse operas than horror. Both films were made during a busy year for Warren – he had three westerns (Trooper Hook, Copper Sky and Ride a Violent Mile) also in release in 1957 – and one can’t help but feel that he might have been stretching himself a little thin with his horrors, as both are lethargic and lacking in that vital spark that can often enliven even the drabbest of under-resourced quickies.

Jim Wheatley (Charles Gray) disappears while exploring the “Cave of the Dead” somewhere in the Caribbean (which looks suspiciously, but perhaps understandably, just like southern California). His sister Gina Matthews (Mala Powers) and her husband Dan (John Howard) come looking for him but are startled by the arrival of Pete Morgan (Paul Richards) who has his eyes and heart set on Gina. They nevertheless agree to take Pete on their expedition and discern a clue in, of all things, a calypso song performed by Sir Lancelot (appearing as himself), “the King of the Calypso”, last seen in a trio of Val Lewton films I Walked with a Zombie (1943), The Ghost Ship (1943) and The Curse of the Cat People (1944). Local villagers deny any knowledge of the cave and seek help from the abusive “Americano doctor” Ramsey (Gerald Milton) and his abused wife Concha (May Wynn) and Gina notices that the fruit he’s bottling has a strange fungus (which reproduces by something referred to as “binary fission”) growing on it. They eventually find the cave where Dan and Pete find several skeletons and they realise that a parasitic fungus is turning the locals into monsters.

It’s a serviceable enough plot, not a particularly original one, but one that could have made for something at least diverting had Warren’s direction been so perfunctory. But worse still, his budget is so negligible that he can’t afford decent special effects and so this dreaded fungus (referred to at least once as “funj-eye”) is realised by slathering extras with… soap suds. Yes, finally the screen quakes to the horror of bubble bath! If you have a phobia about suds, this may well be the most terrifying film you’ve ever seen, though you might be struggling to understand why everyone else is laughing.

In fairness, it’s a pretty poor show even before the monsters turn up but when they do show up, they completely sink an already sluggish and overly talkative affair. Some of the performances are likable enough (Mala Powers is particularly good), but it’s Warren’s inability to make perambulatory soap suds seem even remotely scary that really kills it off. The fact that the film is as (largely) unknown as its terror is perhaps no coincidence. Lacking any real star power, decent monsters or any outrageously silly moments, there’s little here to lodge it in the hearts and minds of even the hardiest of genre completist. It fell by the wayside and has remained there for a long time, unloved and barely known and in honesty it’s fate it probably deserves. The only halfway impressive thing about it are the cave sets by production designer James W. Sullivan which are more realistic and oppressive than you might expect from a film as cash-strapped as this.

Like Back from the Dead, it’s an infuriating film as it has the germ of a really good idea (a parasitic jungle fungus that transforms victims into furious monsters) that neither the budget nor the director is adequate enough to do anything about. A rewrite of Kenneth Higgins’ sloppy script (the basic plot is not dissimilar to that Bert I. Gordon wrote for the same year’s The Cyclops – curiously, Gordon’s Tormented (1960) would be reminiscent of Back from the Dead), a director more in tune with the absurdities of the plot and a cast that believed in the silliness more than this lot seem to might have moulded it into something more enjoyable, but as it stands it’s a pretty dismal show really, recommended only for sudsophobes and lovers of calypso. It remains, at the very best, a footnote in an appendix in the history of 50s American horror films.