Several children go on a lifelong space mission to Cassiopeia because they don't want to go to school.

Interstellar expedition equipped by "pioneers"(soviet scouts) reaches Alpha Kassiopea and finds that smart robots took control of whole planet. Their only goal - to make happy, as they understand, their masters. Happiness was, actually, satisfactions of primitive needs, and removing "disturbing" emotions like love, responsibility etc. A many years ago they succeeded in reaching this goal and all the people died out. A very few people escaped from "total happiness" and their descendants orbiting the planet in a big orbital station for many generations. Brave soviet pioneers land on a planet, destroy the robots and return it to the people