Little Mousen is a big fan of superheroes and an adventure lover. One day, during a family trip, he enters a magical forest and meets a tiger named Tigrulya. It turns out that all the jungle inhabitants are in deadly danger: evil poachers are hunting animals, trying to uncover their immortality secret and find the sacred "heart of the forest." The boy realizes that it's time to introduce the world to a new superhero – Bug Boy!