Pourquoi ?

A man finds a crashed spaceship and reads the log which shows that the alien was to take the form of an earthling when he lands. The man goes to the Space Agency to warn them but the person he talks to (Professor Crater) pulls a gun on him. The man assumes that Professor Crater must be the alien but then finds out that the log is written in another language so he must be the alien and he has helped himself to be caught.