Little Tobia lives with his grandfather Remo, who always tells him fascinating stories. His favorite one tells that at the end of the rainbow you can find a crown of colored diamonds, from which the seven beams of light that form the splendid arch are generated. Tobias manages to convince his grandfather to face the journey to the mountain where he is sure he saw a rainbow spring forth. Tobias, together with his friends Luke, Mack and Ava, the dog Fluffy and the cat Kitty, will set off on a great adventure immersed in nature ruined by man's carelessness.