The game's storyline follows, Nick, who is able to transform into "Captain★Rainbow", a yo-yo-wielding, tokusatsu-styled superhero that stars in his own TV show. Unfortunately, his TV show is no longer popular. To restore his popularity, Nick ventures to Mimin Island, an island where wishes are said to come true. Nick meets a vast array of past, minor Nintendo characters with their own dreams and wishes on his journey.
Captain★Rainbow is an action-adventure game with gameplay divided into two parts. The adventure part of the game is set on enjoying the life on the island together with its other residents. Nick can go fishing, bug catching, and help the other islanders with their requests. Most of the requests can lead to various minigames, such as boxing, volleyball, fishing and golf. Helping islanders will grant the player special, star-shaped crystals known as "Kirarin" (”Sparklies”). The action part of Captain★Rainbow is based around those crystals. Each time 20 Kirarin are collected, a star will fall from the sky. The star can be carried to an altar at the top of the island where it will grant a wish. While carrying the star to the altar Nick is challenged by a mysterious Shadow who for unknown reasons wants the star for himself. If Nick succeeds in bringing the star to the altar there will be two choices left for him. He can either grant his own wish to become a popular hero again or grant the wish of one of the islanders and go back to finding other Kirarin.
Source: Wikipedia, "Captain Rainbow", available under the CC-BY-SA License.