In July 1969, while the world watched Apollo 11 head for the Moon, NASA quietly ran a very different experiment closer to home. Six men sealed themselves inside the PX-15, a cramped submersible capsule, and drifted through the Gulf Stream for 30 days to study how humans cope with isolation and confinement, research meant to inform future long-duration spaceflight. Marine archaeologist James Delgado tracks down the wreck of the capsule decades later and brings together the surviving crew to recall what it was like living shoulder to shoulder in the dark, thousands of feet down, with no control over their course. Interviews with the men mix with underwater footage of the recovered vessel and archival material from the mission itself. The film treats the PX-15 as a forgotten cousin of the space race, a test of endurance run in the ocean instead of orbit, and asks why so few people have ever heard of it.