Unlike Mario Tennis: Power Tour for the GBA, Mario Tennis Open for the 3DS will not include a story mode. According to Camelot it would not have worked because of the inclusion of Miis in the game.
Another reason is that the game’s cast includes Mii characters in addition to characters from the Mario family, which are difficult to work into an [RPG] scenario. It didn’t feel right to have already-existent Mii characters appear in an original storyline. But there are RPG-like elements in the form of costumes. I hope people will enjoy collecting the various costumes to customise their own characters and expand their abilities.
Mario Tennis Open is out May 20th in North America and May 25th in Europe.