Mario & Luigi: Bowser's Inside Story - Plot

Plot

The game opens with a disease called "The Blorbs" spreading across the Mushroom Kingdom. Toads that become infected with the disease inflate to many times their normal size and roll around uncontrollably. A council is immediately called at Princess Peach's castle to discuss what can be done about the epidemic. Starlow, a representative of the Star Sprites that watch over the Mushroom Kingdom, also attends. At the meeting, it is discovered that all those affected had previously eaten a "Blorb Mushroom" given to them by a salesman, who is later revealed to be Fawful. Bowser invades the castle attempting to kidnap Peach, but is defeated by Mario and expelled from the castle.

Bowser later finds himself in Dimble Wood where he is tricked by Fawful into eating a "Vacuum Shroom." The mushroom causes Bowser to mindlessly inhale everything in sight. He inhales Mario, Luigi, Princess Peach, and Starlow before finally passing out. With Peach gone and the majority of the population incapacitated from the disease, Fawful then proceeds to take over Peach's castle while his assistant, Midbus, takes over Bowser's castle. Mario and Luigi, who have shrunken to microscopic size, navigate Bowser's body and manage to revive him. Bowser has no memory of the event and as a result is unaware that the Mario brothers are inside his body. Bowser only communicates with Starlow, who nicknames herself "Chippy" to avoid suspicion. Bowser then proceeds to chase Fawful in an attempt to reclaim his castle with the assistance of the Mario brothers.

Fawful eventually extracts Peach from inside Bowser's body and takes possession of the Dark Star, an evil and powerful entity with a seal that could only be broken by Peach. After a barricade prevents the trio from entering Peach's castle, Mario, Luigi, and Bowser are told by the town doctor that they are required to gather the three Star Cures in order to create the Miracle Cure, a magical medicinal object. The Star Cures are eventually collected and the Miracle Cure destroys the barricade and also heals the Blorbs disease in the process.

Bowser confronts Fawful in Peach's castle and defeats his assistant, Midbus. The Dark Star's seal finally breaks and enters Bowser's body, where it begins to leech on Bowser's cells and absorb his DNA. The Dark Star escapes and is briefly taken back in possession by Fawful, who is then defeated by Bowser. The Dark Star absorbs Fawful's remains and, using Bowser's DNA, becomes a shadowy, powerful doppelgänger of Bowser, named Dark Bowser. Bowser then battles Dark Bowser and, with the help of Mario and Luigi, destroys the Dark Star. Fawful, having been inhaled by Bowser during the battle, initially feigns remorse but suddenly self-destructs in a final effort to destroy the Mario brothers. The explosion causes Bowser to regurgitate everyone he had inhaled. Enraged by the discovery, Bowser proceeds to fight the Mario brothers, but is defeated and retreats back to his castle. The two castles are rebuilt and Peach sends Bowser a cake as gratitude for his unintentionally heroic efforts.