After an evil witch replaces Princess Peach's voice with explosives (causing small explosions to occur whenever the princess speaks), the brothers are once again called to action. Luckily, Peach's explosive vocabulary repels Bowser, who arrives shortly afterward to kidnap Peach. Now the bros. and Bowser have a common enemy, and decide to join forces to get Peach's voice back.
After a short journey with Bowser, the brothers' adventure begins. Encountering enemies and starting battles on the overworld is like Super Mario RPG and Paper Mario; the enemies wander around, and you can choose to either attack or avoid them. Unlike SMRPG and PM, however, Luigi is along for the adventure, and you have control of both brothers at the same time. The A button makes the leader jump, the B button makes the brother in back jump, and pressing Start switches the brothers around. You'll be switching a lot after Mario and Luigi each learn a special jump, which can only be executed by the brother in the back. The special Spin and High jumps let the brothers reach new areas and solve puzzles.
The one-button-per-brother system carries over into battle. During a battle, the A button controls Mario's offense and defense, and the B button controls Luigi. Timed attacks return from SMRPG and PM, where pressing A or B at the right time during an attack causes more damage to enemies, while a well-timed button-press during an enemy's attack lets the brothers avoid or counter it. Later on, the brothers learn "Bros. Attacks"; acrobatic attacks performed in tandem, requiring three "timed attacks" to dish out maximum damage.
The Mario Bros. mode from the Super Mario Advance series is also included.
Despite the similarities to SMRPG and Paper Mario, Mario & Luigi: Superstar Saga has less RPG elements than both. Hovever, the bizarre story and innovative battle system creates a fresh gaming experience that is sure to please Mario fans new and old.