Blue's Clues Season 3 Episode 21 Blue's Big Mystery Episode Title:Blue's Big Mystery.Episode Description: There's somebody building a fort in the backyard of the Blue's Clues house, but we have no idea who it is. In this episode, we put on our detective caps and logical-thinking skills. There's a mystery to be solved. Come along as we investigate, with a little help from Mr. Salt and all of our friends.