On board the tramp steamer Corita, Captain Lansen, Eric Porter, ignores a hurricane warning and a customs launch that would inspect his ship, because he is smuggling the explosive Phosphor B (Phosphore Blanc, i.e. white phosphorus). His passengers—a cross-section of the wealthy and the poor—each have their own reasons for leaving Freetown: Dr. Webster, [Nigel Stock], and his daughter Unity, [Suzanna Leigh], due to his indiscretions with patients; alcoholic conman Harry Tyler, with a jacket lined with money; Eva Peters, a wife who has stolen bearer bonds to pay a ransom on her son in Caracas; and lawyer Ricaldi, pursuing Eva to recover those stolen bonds.
An accident nearly triggers the explosives, and tensions peak when an anchor chain slips a gear, ruptures the bulkhead, and power is lost. Some of the crew mutiny and flee in a lifeboat. Chief Engineer Nick, [James Cossins], cannot repair the generator, and Captain Lansen, fearing the ship may explode as Hurricane Wendy closes in, leads the remaining crew and passengers into another lifeboat. In the ensuing chaos, Dr. Webster is devoured by a shark.
The following morning, Lansen's lifeboat drifts into a morass of sentient, carnivorous seaweed that claims the cook. The lifeboat later bumps into Corita again, its propellers fouled by the pullulating seaweed, yet the survivors must retreat back aboard the doomed vessel. That night, Unity is attacked by a glowing-eyed prehistoric octopus (an over-sized Keuppia), which kills Ricaldi when he tries to rescue her.
Sarah, a native girl from a nearby island, appears, walking atop the seaweed and kept from sinking by air buoyancy balloons and pads on her feet. She is pursued by fur-clad barbarians but warns Captain Lansen of an impending assault. The crew and passengers defend the Corita, and the barbarians retreat to a dilapidated Spanish galleon marooned nearby. The boy-king “El Supremo,” descendant of Spanish conquistadors, along with members of the Spanish Inquisition, had ordered the attack to steal their supplies.
Sarah attempts to return to her island but is tracked by the conquistadors. While guarding an outcrop, the bartender is killed by a giant hermit crab, which is then attacked by a giant Paleo Scorpion. Patrick, the bartender, [Jimmy Hanley], is killed, and Sarah, Harry, and the ship’s engineer are captured by the Spanish, taken to the galleon and brought before El Supremo, while it becomes obvious that the masked Inquisitor is effectively calling the shots.
Captain Lansen confronts them, insisting they will not yield or abandon their journey home. He uses the explosives to destroy the galleon and rescue his crew and passengers. Inspired by Lansen’s defiance, El Supremo abandons his throne to join them, only to be stabbed by the cleric the moment he turns his back. Stripped of his power, the plague-riddled priest kneels in prayer as the monks cry out and flames erupt around them.
Lansen and his survivors, along with those Spaniards who chose to stand with them, return to the Corita. The shrouded cadaver of El Supremo—never to recover from his wound—receives a burial at sea, while Lansen reads the Rites from the Book of Common Prayer.
