So according to the introduction of the story, E. Gadd used to own friendly ghosts that worked for him in his lab until the Dark Moon, which I assume is a power source, somehow spilt and caused the ghosts to become mischievous and rebel against E. Gadd. The professor contacts Luigi to help him capture the ghosts and restore the Dark Moon, and the adventure begins.
I believe King Boo, who was somehow freed from his painting, got revenge on E. Gadd by breaking the Dark Moon in an effort to free the ghosts from E. Gadd's control just as the portrait ghosts from the first Luigi's Mansion were freed from E. Gadd with King Boo's help. And it seems like Mario is not kidnapped again, so thankfully there's not another damsel-in-distress plotline.





