Captain Lansen (Eric Porter) reads the Burial Rites from the Book of Common Prayer watched mournfully by a motley crew of seamen, pirates, Spanish ladies, armoured conquistadors and priests (all seemingly from different time periods). As the shrouded cadaver is slid overboard from beneath a flag the captain asks: "What happened to us? How did we all get here...?"
... On board the tramp steamer Corita, Captain Lansen first ignores a hurricane warning then ignores a customs launch whose crew want to inspect his ship, as he is smuggling the explosive "Phosphor B" ('Phosphore Blanc', i.e. white phosphorus). His ship's passengers, a mix of rich and poor, also have various unwholesome reasons for leaving Freetown: Dr. Webster (Nigel Stock), together with his daughter Unity (Suzanna Leigh), for his indiscretions with patients; an alcoholic conman Harry Tyler (Tony Beckley) with a jacket lined with money; Eva Peters (Hildegard Knef), a trophy wife who has stolen bearer bonds to pay for the ransom on her son in Caracas, and lawyer, Ricaldi (Ben Carruthers), pursuing Eva Peters in order to retrieve those stolen bonds.
An accident nearly sets off the explosives and the authority of the captain comes to a head when an anchor chain slips a gear, ruptures the bulkhead and power is lost. Some of the crew mutiny and take to a lifeboat. Chief Engineer Nick (James Cossins) cannot repair the generator and Captain Lansen, fearing the ship may explode with Hurricane Wendy about to engulf them in a storm, takes the remaining crew and passengers into another lifeboat. In the ensuing chaos, Dr. Webster is soon devoured by an attacking shark.
The following morning, Lansen's lifeboat finds itself adrift in a morass of large, sentient and carnivorous seaweed, which kills the cook. The lifeboat later bumps into the Corita again, and its passengers find the propellers of the tramp steamer completely fouled by the pullulating seaweed. Nevertheless, those in the lifeboat are forced to take refuge in the doomed vessel once more. That night, Unity is attacked by a huge glowing-eyed octopus, which kills Ricaldi when he attempts to rescue her.
Sarah (Dana Gillespie), a mysterious native girl from a nearby island, suddenly appears, walking on the morass of seaweed, prevented from sinking by air buoyancy balloons attached to her shoulders and odd pads attached to her feet. She is being chased by a bunch of fur-clad barbarians but she warns Captain Lansen of an impending attack. The crew and passengers defend the Corita, with the surviving fur-clad attackers returning to a dilapidated large wooden Spanish galleon, marooned nearby. Child leader "El Supremo" (Darryl Read), the princely descendant of the Spanish Conquistadors, along with members of the Spanish Inquisition had ordered the attack in order to steal their supplies.
Sarah attempts to return to her island but is tracked down by the Spanish conquistadors. Whilst standing guard, on an outcrop of rock, the bartender (Jimmy Hanley) is killed by a giant hermit crab, which in turn is attacked by a giant scorpion. Sarah, Tyler and the ship's engineer are then captured by the Spanish, taken to the galleon and brought before "El Supremo", though it is soon obvious to all that the one standing aside and clad in ominous and 'pointy-hatted' capirote Inquisitor's clothing, is "calling all the shots" (in the guise of God's Will). Captain Lansen confronts him, stating that they will not give in to his demands or ever stop attempting to get back home. Lansen then uses the explosives to destroy the galleon and rescue his crew/passengers. Inspired by Lansen's speech and attitude, the child leader "El Supremo" then abandons his throne to join them, only to be stabbed by the traitorous cleric the moment he turns his back to leave. Stripped of his power, the plague-riddled priest succumbs to kneeling and prayer, while his monks succumb to frantic pipe organ-playing as the hell of flames erupts all around them, creeping ever closer to the sounds of liturgical hymns and chaos.
Captain Lansen and his crew, along with those Spaniards who have decided to join them, head back to the Corita and we return to the start of the film ... with the burial at sea now revealed to be the funeral of the child leader "El Supremo", who never recovered from his stab wound. His sad death in Eva's arms previously, had reminded her of her own son's perilous predicament and uncertain fate - if they ever find their way home?
