Upon returning to the amusement park planet, the Enterprise crew finds the computer, which generates the planet's sophisticated robots, running amok, with the caretaker nowhere to be found.
Exposed to spiroid epsilon waves, the Enterprise crew begins to shrink in size. If a way can not be found to stop their condition, they will soon be too small to control the ship.
While the Enterprise investigates a water-world, Captain Kirk and Commander Spock go missing. They've been abducted by the world's inhabitants and altered into water-breathers as well.