A member of the Council of Science, Lou Evans, a troubleshooter on Venus, is to be removed from Venus with a charge of corruption. Lucky Starr had roomed with Lou when being trained requests going to Venus to help clear Lou's name. The planet Venus, was found to be covered by water. The humans that now make it home live under the water in bubbled cities. The surface of the water on the planet is fully covered with seaweed. On the seaweed live V-frogs, amphibians that humans have taken down into the domes as pets. Lucky and Bigman are almost killed when the pilots of their ship pass out and crash land into the seaweed. Another human then threatens the entire city by almost opening a hatch that would let in the water into the city in a flood. When Lucky confronts Lou, Lou acts like he wants help, but then seems to change his mind and tell Lucky to leave him alone. When Lou escapes from the underwater city, Lucky and Bigman go after Lou. Lucky finally figures out what is going on and why humans are passing out or acting strange. Now that Lucky knows who the enemy is, he find himself under attack too . . .