The film opens with a crew of pirates lounging after a raid, until one curious pirate slips away to explore a cave and stumbles into a concealed tunnel network that burrows beneath the island.
Warren Price, [Les Loveday], a millionaire’s son, leads a dangerous expedition to the former Japanese military bunker on Una-Una, Gulf of Tomini, Sulawesi, accompanied by Japanese scientist Rie [Miki Mizuno] and Price’s girlfriend Su-Ling [Carmen Soo]. They are escorted by a skilled team including Captain Tino Prawa [Ario Bayu], Djoko [Joe Taslim], Ario [Mike Lewis], and Sergeant Papa Snake [Yanda Djaitov], with Stanley [Sam Hazeldine], a former military engineer turned mercenary, joining them on the mission.
The group arrives at the site of a Dutch-operated mine that the Japanese had seized in 1942 and transformed into a bunker. After an ambush by pirates, they’re forced inside, and a pirate’s grenade into the mine entrance collapses the way out, injuring Ario and trapping them underground. Desperate to escape, they descend deeper into the mine, while Price’s true goal comes into view: he hopes to unearth Yamashita’s gold. He and Rie have been trying to decode coded messages from a previous expedition near Sumatra, and Price believes the treasure is tied to a mutagenic formula buried somewhere in the bunker.
As they press on, lights flare and a Rising Sun flag appears on a wall, and a haunting propaganda song crackles through the comms, waking a sense of dread. The group suspects there is a second entrance and bracingly continues through the abandoned Japanese base, discovering Hazmat suits, old maps, and disturbing photographs. Outside the communications center, Papa Snake spots a figure wearing a Hazmat suit, but the apparition disappears as soon as he looks away.
Their investigation reveals that the bunker was used for chemical and biological weapons research, influenced by Unit 731, where prisoners of war were subjected to horrific experiments. The party is stalked by hazmat-clad figures as they move through the tunnels. They split into two groups: Captain Prawa, Papa Snake, Price, and Su-Ling search for a way out (and for the gold), while Djoko, Stanley, and Rie stay behind to guard Ario.
Ario, resting on a pallet, is suddenly attacked by a monstrous creature and dragged into the tunnels. Djoko, Stanley, and Rie race after him, but danger closes in. In a chamber of human remains, Djoko discovers the mutilated pirate from the opening and fights one creature, only to be outmatched as more emerge; his rifle is lost and his pistol jams, leaving him vulnerable to the advancing horrors.
Meanwhile, Rie and Stanley are captured by a gas-masked Japanese soldier who refuses to admit the war is over. Stanley manages to disarm him, and Rie shows footage of the Japanese surrender on the USS Missouri, breaking the soldier’s resolve. Captain Prawa’s squad is then assaulted by another creature; Price is wounded, and a shot fired in the fray barely keeps him alive as Papa Snake kills the creature with an improvised spear. The Japanese soldier reveals that a gas used in Unit 731 experiments drove prisoners insane and mutated them into monsters, and that his own soldiers were mutated into “super soldiers” as part of the project.
Price discovers that the gold had been shipped to the base to fuel the mutagenic formulas given to prisoners. Desperate to profit, he and Su-Ling attempt to smuggle a sample out, hoping to sell it on the black market as a deadly biological weapon. Captain Prawa and Papa Snake stumble upon a shrine filled with dozens of creatures wearing samurai armor—the Imperial Guards, soldiers who were subjected to the same mutagenic process. Papa Snake stays behind to hold them off and is fatally stabbed, while Captain Prawa, Price, and Su-Ling continue fleeing and stumble upon Djoko’s corpse, only to be attacked again by the mutated dwellers. They fight with improvised knives and manage to reunite with Stanley and Rie.
Price is badly wounded, and Su-Ling administers the mutagenic formula they’ve recovered to try to save him. The Imperial Guardsmen flood the tunnels, killing the gas-masked Japanese soldier and forcing the survivors to scramble for safety. The mutated former POWs, now allied with the Imperial Guards, close in. In the chaos, Su-Ling abandons the others, dragging Price through a doorway and locking it behind her.
Captain Prawa sacrifices himself in a heroic last stand to buy time for Rie and Stanley to escape. Su-Ling drags Warren Price to the cave’s edge, but the mutagenic dose drives him insane, and he kills her in a brutal, final betrayal. Rie and Stanley flee the mine through an underwater tunnel system, with only Rie managing to reach safety. As more Imperial Guardsmen flood the lake, Rie defends herself against the advancing swordsmen, but the end comes when a guard’s blade swings into her midsection, and the film ends with her fate left hanging.
