A reflective statue is found at the bottom of one of Earth's oceans, having lain there for 1.5 billion years. Since humans have recently developed a time-slowing field and found that one such field cannot function within another, it is suspected that the "Sea Statue" is actually a space traveler within one of these time fields. Larry Greenberg, a telepath, agrees to participate in an experiment: a time-slowing field is generated around both Greenberg and the statue, shutting off the stasis field and revealing Kzanol. Kzanol is a living Thrint, a member of a telepathic race that once ruled the galaxy through mind control.
Eons ago, Kzanol's spaceship had suffered a catastrophic failure; its reactive drive system failed and the navigation computer automatically jettisoned it. Faced with insufficient power to use hyperspace, Kzanol aimed himself at the nearest uninhabited Thrint planet (which turns out to be Earth) used to grow yeast for food, and turned his spacesuit's emergency stasis field on to survive the long journey and impact. He also arranged for his ship to change course for the system's eighth planet (Neptune) after he was in stasis, with his amplifier helmet and other valuables inside his spare suit (in order to hide these valuables from any rescuers).
Although he assumed that the resident thrint overseer would be able to rescue him after seeing the plume of gas created by his impact, his timing could not have been worse; while in stasis on the way to the planet, the slave races revolted against the Thrint. Facing extinction, the Thrint decided to take their enemies with them by constructing a telepathic amplifier powerful enough to command all sentient species in the galaxy to commit suicide. They set it to repeat for centuries and every sentient being in the galaxy perished. After hundreds of millions of years, the yeast food mutated and evolved into complex life on Earth.
After his telepathic encounter with the Thrint, Greenberg is confused by having two sets of memories, his own and Kzanol's. He instinctively assumes he is Kzanol. Both Greenberg and the real Kzanol steal spaceships and race to reclaim the thought-amplifying machine on Neptune, which is powerful enough to enable control of every thinking being in the Solar System. Eventually, Greenberg's personality reasserts itself and, armed with the knowledge of how to resist the Power (one of Kzanol's own memories), Greenberg traps Kzanol again in a stasis field.