Inventor Fred Miller is determined to prove to his old-fashioned boss, T. R. Hollister, the feasibility of his newly-designed undersea house, which he calls the Green Onion, by moving his family underwater for 30 days. He first overcomes his wife Vivian's fear of water and then agrees to let his teenage son and daughter, Tommie and Lorrie, bring along the rest of their rock and roll band so that they can practice; a potential contract with Nate Ashbury, boy wonder of the record industry, hangs in the balance. At first all goes well for the group as they are joined by a friendly seal, and two friendly dolphins help them ward off such dangers to underwater living as menacing sharks. Eventually, however, trouble erupts when Fred's company rival, Mel Cheever, begins construction nearby on a deep sea mining machine, and the rock music creates strange sonar blips which the Navy attributes to an enemy power. Chaos sets in when Cheever siphons a tank full of gas from Fred's house, thereby causing it to tilt and throw the electrical appliances into reverse. Cheever's experiment turns into a disaster, however, and he has to be rescued by Fred. Ashbury likes the new music created by the teenagers and signs them for the Merv Griffin television show; but in their eagerness to reach land in the family submarine they are stranded in a hurricane. Once again, Fred and the dolphins go to the rescue. All ends happily: Hollister approves the house, Merv Griffin tapes the rock group underwater, and the Navy discovers that the only thing they have to fear under the sea is the Green Onion.