In his unique invention the Ocean Arrow, an "underwater helicopter," Tom Swift embarks on a perilous search for one of history's greatest scientific treasures -- a lost rocket bearing life from outer space. Directed to Swift Enterprises but diverted from its course by a treacherous rival, the vessel has plunged into the depths of the Atlantic, its location unknown. Accompanied by his friends Bud Barclay and Chow Winkler and guided by a pair of expert oceanographers, the young inventor sets out in his diving seacopter to explore the seafloor near the Madeira Islands. But the enemy team has set forth in their own craft to thwart Tom in his efforts and claim the historic space treasure for themselves. Violent storms and the discovery of a lost subocean city are preludes to the terrifying moment Tom is shipwrecked far beneath the sea!