Nothing foretold the trouble on planet "M". Every day the Sun was rising and setting down at the same time. The colourful houses, by shape look like beans, charged from the Sun's energy. At night, they rose upward and gave off their light like flashlights. Nevertheless, suddenly the monsters woke up in the pitch darkness. In the morning, the houses went down on the blue grass but the Sun was not shining. What is the matter? The houses cannot rise upwards to illuminate planet "M" without the Sun. The monsters began to turn on flashlights on their gadgets. The watchful monster Cynthia has noticed footprints in the twilight. She has scanned that finding with her magnifying glass. These are the evil monster Henry's footprints, are not they? Sean and Brad are sure that Henry is involved in the disappearance of the Sun. Where will they search the Sun? No one knows that the Sun disappeared from planet "M" once, before the birth of monsters. But where? Moreover, who returned it back? Suddenly Brad's sensor starts giving a loud signal - his little sister MiMi has disappeared. The sensor indicates that she is far beyond the city of "I", where Monsters-friends have never been before. Monsters go to seek Mimi without wasting any time. Will the friends find MiMi? Will they manage to return the Sun back?