Watch Captain Fathom for Free: How to Stream the Movie Online at 123Movies. . While traveling in enemy waters, Captain Fathom and his crew detect an anomalous reading of radioactivity. The submariners investigate and discover a huge bomb lying on the ocean bottom. While Fathom fastens an explosive to destroy the atomic device, his ship is attacked by an enemy submarine. He orders the vessel to flee leaving the Captain and his team of scuba divers behind with a dwindling supply of oxygen in their tanks.