The story opens in a Western saloon, where a young musician with a banjo begins to tell a tale of Lucky Luke and his sworn enemies the Dalton brothers: Joe, William, Jack and Averell. Luke has, once again, as he has done many times before, thrown the four outlaws into jail. The prison is also the abode of a guard dog named Rin Tin Can (Rantanplan in the original French language version).
Zillionair playboy Terry has built a spaceship so that he can see the galaxy together with his girlfriend Ferma. Along comes a spaced-out alien who needs to find back to his planet. The trio brings along Little Orbit the Astrodog to assist in the search. On their fantastical journey they battle a phantom space-bird, asparagus monsters (with Bronx accents) that want them for lunch, and a race of space-robots on a machine planet.
