"One blissful day in the Mushroom Kingdom, genius inventor Professor E. Gadd constructed a time machine, which was fueled by the awesome power of the Cobalt Star. Princess Peach was overjoyed with his creation, and immediately set off to visit the Mushroom Kingdom's past. Of course, her faithful ward Toadsworth was filled with such worry for his charge that he promptly collapsed."
"Shortly thereafter the time machine returned, but it was a wreck and it brought back not the princess, but a creepy alien of unknown origin. According to the professor, Princess Peach was stranded somewhere in the past! To top it off, mysterious time holes leading back in time began popping throughout the castle! Who better to answer the call to adventure than Mario and Luigi? Nobody, that's who!"
"The mustachioed brothers leaped into action, plunged through a time hole, and set off on a rescue mission through time. Once they arrived in the past, the brothers met up with their childhood selves and formed the greatest team of heroes in history. Can the Mario brothers overcome all obstacles and rescue Princess Peach? Only time will tell!"
The gameplay is similar to the previous title in the series, though with some extras included. First off and unarguably the most notable extra is the fact that you control four characters while fighting and while on the main world, compared to just two in Mario & Luigi: Superstar Saga. These two characters happen to be Mario and Luigi's baby counterparts from the past, who team up with the brothers to stop an alien invasion.