Guys this except from IGN explains some aspects of the Game.
Captain Rainbow tells the story of a hero of the same name and his every day alter-ego Nick. Rainbow is the star of a television show in the land of the free, Mameruka (this could be a pun on "America"). Thanks to the appearance of some unpopular characters, the ratings for his show have tanked. In order to gain back the show's popularity, Nick decides to head off to Mimin Island. Something happens along the way, however, and Nick learns that he is the only person who can help out the residents of Mimin.
And those residents happen to be the second tier characters from the Nintendo family. Actually, maybe third tier is a more accurate description. The characters you encounter here may be recognizable by only the most hardcore of Nintendo fans. Would you recognize Ossan, the character from the original Famicom Golf? How about Hikari, from Famicom Mukashibanashi (this latter title was only available in Japan)? More people, at least in America, should be familiar with Catharine, the Super Mario Bros. 2 boss who spits out eggs at you.
It's a bit unclear what you'll be doing as you take control of Nick and Captain Rainbow and meet all these characters. While the game offers a variety of activities like fishing, bug collecting, and golf, the actual point of the title is being kept under wraps. There seems to be a side scrolling action component to the game, but it's possible that this is just one aspect of the game. -----IGN
Link:
http://wii.ign.com/articles/887/887354p1.html