Gerald McBoing Boing, a little boy who can't talk but can imitate any sound, is working as a one-man sound effects department for a radio station. When a scheduled symphony orchestra does not show up, Gerald replaces them. He is doing fine until he mixes up the score with the sound effects script, and creates an original symphony. He is fired, but the critics hail his work as that of a genius.

While meeting a new friend, Gerald is abducted by aliens and whisked to the planet Moo. The king of Moo mistakenly thinks that all Earthlings - like Gerald - speak only in sound effects, and he attempts to converse with Gerald. Hoping to lure Earth tourism to his planet, the king brings the boy back to Earth in the hope of establishing good relations, but Earth diplomats are puzzled by the king's unusual language