Luigi is resting at his home when Mario comes in and tells him something horrible has happened. Luigi steps outside and sees a giant ghost octopus has risen up out of the ground and is destroying the Kingdom. Luigi and Mario must go through and evacuate the kingdom, going through burning buildings and underground tunnels. Afterward, E. Gadd gives Luigi his Poltergust 4000, and Luigi fights the giant octopus. After going through and sucking in the tips of all the tentacles, weakening the ghost, Luigi must jump on top of it and suck in all the tiny Boos floating around it. Finally, Luigi jumps back down to the ground, puts the Poltergust on "Reverse", and begins shooting the Octopus ghost with Boos until it finally dies and fades away. Peach tells Luigi and Mario that there is an infection going on, turning all citizens into flying intangible spirits called "Shroombies". Luigi and Mario set out to find the ancient Shroomsquatch, a giant ape-like creature living in the forest whose magic fur can cure any disease.