A red alert is signaled after a supply ship's concrete mooring, cast off during a hurricane, unwittingly strikes Sealab's control dome and splits a major section of the shell.
A new world opens up for Roger Cummings, a disabled boy whose father is doing research at Sealab, when Bobby befriends him and teaches him how to dive.
Human complications make dealing with the mechanical ones more difficult when cargo sub "Aquarius" crashes into Seamount and endangers the Sealab complex.
Hal, Ed, and Gail plunge to the lowest depths of the ocean in a special crystal bell to plant a second seismographic signaling device after the first one mysteriously disappears.
When a space probe loses its parachute during reentry and plummets out of sight into the ocean's depths, the staff of Sealab is called on to locate it.