Official story and features:
"The stars of the Mushroom Kingdom are gathering for a tennis tournament, and the competition is going to be fierce!
Choose your friends wisely. Power characters like Donkey Kong and Bowser can hit insanely powerful Megaton Balls, finesse players like Shy Guy can use instant Tornado Returns and defensive masters like Luigi have all kinds of tricks in their bags!
With one- to four-player modes available, players of all ages can pick up the game and instantly start whacking tennis balls around, and yet a deep level of skill awaits those willing to master it. Build up more and more skill points as you play to use stronger and stronger shots with all kinds of spin!"
From Nintendo site