In the Pacific Ocean in during World War I, a German U-Boat carrying a gold shipment comes across a British sail ship. The German U-Boat commander, Von Boulten, has the U- Boat surface only to discover that the British ship has hidden guns aboard. In the battle, the German U-Boat is sunk, and Von Boulten and his lieutenant, the sole survivors, swim ashore. When their rescue seems assured, Von Boulten pushes his mate down a cliff, keeping for himself the map with the gold's location. Twelve years later on the docks of San Francisco, Von Boulten, now named Karl Schlemmer, meets with deep-sea diver Steve McCreary. Along with Lily, the group's financial backer, the three make a pact to raise the $3,000,000 in gold, and split it three ways. Out at sea, Schlemmer faces a near mutiny by his crew due to the rough weather. Steve tries to force Schlemmer to give him the map, but Schlemmer pulls a gun just as the ship is sunk by a giant wave. Together on a lifeboat, Steve tears the map in half, effectively making himself Schlemmer's partner. Three years later, Steve and Schlemmer sign on with a scientific expedition financed by socialite Diana Templeton, an aspiring oceanographer. Steve and Diana are attracted to each other, though Steve sees her mostly as a publicity-seeking nuisance, more interested in having her picture taken than science. When Diana gets Steve alone in the ship's dark room, he kisses her and becomes hooked. Lily, who was left behind in San Francisco, stows away on board and becomes a member of the galley crew. Schlemmer promises each partner that he will cheat the other, though Steve realizes that Schlemmer wants all the gold for himself and tells him so. When Diana uses Steve's diving gear without his permission, she develops the "bends," and Steve must rush her to the decompression tank. At dinner that night, scientists Dr. Chapman and Horace Waldridge want Steve removed from the ship, much to Schlemmer's chagrin. Steve arrives drunk and tells the group off, but Diana, surprisingly, takes all the blame herself. On deck, Steve and Diana make up. The next day, Steve and Schlemmer take off in a tender. After discovering the sunken U-Boat, Steve uses an underwater torch to cut inside and find the gold box. They attach the gold to a buoy, planning to return the next day with a crane to recover it. The next morning, Schlemmer and Lily drug Steve's coffee and leaves him behind. When he awakes, he discovers the two are gone. Diana and photographer Bert Jackson, meanwhile, go down in the diving bell, which is attacked by a giant octopus, which breaks the bell's air hose and steel chain. Steve goes down in his diving suit and kills the octopus with the torch, and the bell is raised just in time to save Diana and Bert from suffocation. Steve spies Schlemmer and Lily as they raise the sunken treasure. However, the box breaks just as it surfaces, and the gold spills out. As the box falls into the water, Schlemmer's foot becomes tangled in the chain, dragging him down with the gold. On deck that night, Steve tells Diana that saving her cost him $1,000,000, as the gold, now in the muddy sea bottom, is lost forever. He tells her she is just trouble, like him, and they kiss.