Impostor

"Impostor" is a science fiction short story by American writer Philip K. Dick. It was first published in Astounding SF magazine in June 1953. Spence Olham, a member of a team designing an offensive weapon to destroy invading aliens known as the Outspacers, is confronted by a colleague and accused by security officer Major Peters of being an android impostor designed to sabotage Earth's defenses. The impostor's spaceship was damaged and has crashed just outside the city. The android is supposed to detonate a bomb on the utterance of a deadly code phrase. Olham, in an attempt to clear his name and prove his humanity, manages to escape his captors and return to Earth after they fail to kill him on the Moon.