The Master Suite is the fifteenth floor of The Last Resort in Luigi's Mansion 3 and it is the only floor not to contain a hidden Boo. It can be accessed after recovering this floor's elevator button by capturing DJ Phantasmagloria in The Dance Hall. It is the highest floor of the hotel and Hellen Gravely's personal quarters. Due to her fondness of King Boo, several of the objects and furniture in the Master Suite resemble him, including lamps, trash cans, pillows, and slippers. Paintings and sculptures of King Boo are found in the main hallway, and the gems on this floor are also shaped like him. As soon as Luigi enters the Master Suite, a video of Hellen appears on the projection screen. She congratulates Luigi for making it this far, but warns him that it is not over yet. She then reveals that she plans to capture Luigi herself and give him to King Boo as a present. She also reveals that Mario's painting is hanging above her desk. After the video, Professor E. Gadd calls Luigi and tells him to face off against the hotel owner and rescue his brother.
There is a pipe in the southeast corner of the Lounge which takes Gooigi up to a chain, which he must pull to raise the projection screen, revealing a hidden door leading to the main hallway. In order to reach the Main Observation Room where Hellen is, Luigi must find four keys and insert them into the tongue of a large sculpture of King Boo at the far north end of the hallway. The keys have tops shaped like King Boo's Crown and are colored pink, green, blue, and yellow, the same colors as the keys guarded by area bosses in Luigi's Mansion. Each key is in one of four rooms connected to the hallway. The rooms feature laser security systems which Luigi must avoid and disarm to obtain the keys.
Upon reaching her, Hellen berates Luigi for messing up her hotel and capturing her staff, as well as capturing her ghost cat, Polterkitty. She reveals her true form and shuts the vault, challenging Luigi to a battle. Her battle consists of Luigi and Gooigi working together to reroute lasers to reach her. Once she has been captured, Luigi can use the Dark-Light Device on Mario's painting to rescue him. He and Luigi hug and rejoice. Just before Luigi leaves, Mario shows him a door hidden behind the screens. This door leads them up to the Rooftop, where Luigi is able to rescue Princess Peach and battle King Boo. Before going through the door, Professor E. Gadd warns Luigi of an unusual presence behind it, and asks if he's ready to keep going. If he declines, E. Gadd advises him to do what he feels is best, but recommends that he prepare himself; if he says "Yes", E. Gadd wishes him the best of luck, but then gets captured by King Boo.