The weirdest alien of the galaxy pays a visit to Earth... Jerry Lewis is Kreton, a childish alien who, against his teacher's will leaves his planet to visit the Earth, and lands in the backyard of a famous television journalist who doesn't believe in UFOs and aliens. Wanting to study humans but not able to fully understand them, Kreton makes a mess out of it, generating a lot of comic situations.

When a crooked sheriff murders his employer, William "Billy the Kid" Bonney decides to avenge the death by killing the man responsible, throwing the lives of everyone around him into turmoil, and endangering the General Amnesty set up by Governor Wallace to bring peace to the New Mexico Territory.