Coeurl, a catlike creature that is nearly immortal, is on the hunt for id-creatures so that it may sustain itself. While traveling across its home planet, it realizes that it has hunted every possible victim to extinction. Then suddenly, something arrives. It is a spaceship full of humans who are bursting with id. Coeurl runs toward them, but remembers the violence that destroyed his planet and ambles close to them instead. They are scientists there to investigate the ruined city and planet, and believe Coeurl may be a descendant of the alien race that ruled it long ago. They believe that it can only breathe chlorine gas as that was the predominant gas on the planet, but when Coeurl enters the scientists' spaceship just fine, they are all stunned and decide to keep it around for study. Little do they know Coeurl was there only to kill them all and take their spaceship back to their home planet for more id.

And kill he did, slowly at first. Starting with one lone man who was sent deep into the city to investigate. Coeurl is able to find and kill him within only minutes, getting back to the group of scientists who are none the wiser to his disappearance. When they find the body, only one person suspects the cat-like alien. The others are worried, but won't jump to any conclusions just yet. An autopsy reveals that the body has been cleared of all phosphorus, and the most vocal against Coeurl places a bowl of it in front of it to see how it reacts. Coeurl knows it's about to be found out and fights any urge in his body to react, but throws the bowl at the man instead. Coeurl is almost killed, but the captain of the ship intervenes and saves its life. The captain tricks Coeurl into going into the brig, where it is locked away full of rage and plots his next kill. Using its innate ability to read the vibrations of materials, Coeurl matches the vibrations of the electronic lock keeping its cell closed and opens the door.

Coeurl is suddenly lonely as the only member of his species on this ship fighting against these 100 men, but the series of murders it commits abruptly override any other emotions. All it wants to do is kill. But when the guards realize his door is open, the cat runs back, kills the guards, and shuts and locks the door behind itself again. The captain believes that Coeurl is innocent and wants all of his men tested for space madness. But they realize that Coeurl could have used his vibration powers to open the door, and when they try to kill it by electrocuting the floor, it overrides the circuits with his powers and saves himself. But now it's angry and locked in a dark cage, about to open the door and escape to try and kill more men. Using its vibration powers on the back wall, Coeurl escapes into the engine room and turns them on, sending the ship into space and locking the door behind it.

The crew frets over this creature they cannot understand, but assume desires only galactic domination. The captain comes up with a convoluted plan that involves shooting down the four doors into the engine room and cutting all power except for the engines. They discovered exactly which machine Coeurl likes the most in the room, and the captain pulls everyone back for one final regrouping and a new attempt against the cat monster. But, Coeurl welds the walls and doors together using its vibration powers and turns the engine room into its own spaceship. Thinking it is heading back to its own planet to help the others of its species create spaceships to help take over the galaxy, it instead discovers that the original spaceship took over the controls and turned his ship around. Coeurl kills itself rather than be captured, and the men resolve to commit genocide against every other member of its species on the planet, no matter how primitive they may be, so that none may take over the galaxy.