Throw the Mario Bros. into a timehole, their younger selves, hilarious dialogue, and alien shrooms, and you have a win.
The story starts out with Princess Peach boarding a time machine built by Prof. E. Gadd (who also played a guest appearance in Superstar Saga), and goes back in time to see the past Mushroom Kingdom. Mario and Luigi go to the castle to greet the Princess when she returns, only to find a battered-and-bandaged time machine with a purple alien inside! To save the Princess, Mario and brother Luigi jump into a timehole and into the past, soon meeting their baby versions. Thus they head out into an incredible "Back to the Past then Future then Past again" quest!
The graphics in this game are a touch smoother and glossier than Saga. Everything is a 2D-3D look like Saga was, as well. Not much has changed beyond that.
Combat is also reminescent of Saga, with the addition of the Baby Bros. using the X and Y buttons. Plus, when the Adults make an attack, if you press the respective Baby's button at the right time, they join in and deal additional damage.
Another change to combat is the way combo attacks work. Instead of using Bros. Points, you use special items (for example a cannon, just like in the funny commercial), which starts the attack. But just like Bros. Attacks, you have to use button sequences.
In Saga, if you recall, you could also team up on the field. The Adults get their Spin and High jumps, plus the ability to roll into a ball. The babies, on the other hand, get the hammers in the game (makes you wonder what's going on in Japan's household), which they use to pound eachother with, going into the ground or into a pancake(???). What a way to solve puzzles.
This is a must buy if you have a DS and enjoyed Superstar Saga. Just remember to turn the volume down when Baby Peach comes onscreen.