Professor Bulfinch builds an incredible laboratory-ship that can explore the ocean floor, and he asks Danny and his friends to accompany him when he tests it!

As they travel thousands of feet under water, Danny, Irene, and Joe conduct their own experiment. Using a special recorder, they try to tape the sounds of many fish, form the tiny glittering hatchetfish to the giant squid.

But soon they run into trouble; their tank pumps break down and they sink to the very bottom of the ocean - trapped in an undersea canyon! Then a nearby cliff collapses. As sand and mud cloud the water, they can no longer see their way. Can they escape before their food and air run out?