1F: The Courtyard
Return to the Mansion. Go to the very back of the corridor on the first floor, to a glowing, locked door. As you approach it, it will shine, and the spikes blocking it will disappear, making it safe to open. This door unlocks with the Key of Clubs you received after defeating Bogmire. Out here, you don't have much to do. You can water some of the plants for extra cash. (One contains a green jewel.) Check the birdhouse to find Mario's Letter, another of Mario's lost items. There are three Purple Bombers, and one Ceiling Surprise hanging from the trees. Catch them to make a chest appear, containing a lot of money. Near the door at the east side of the garden is a small wooden shed. Open it by hitting it from behind. Speak to the Toad inside. 'Something terrible is inside that well...' Go down the well beside the treasure chest.
BF: Bottom of the Well
Go to the right once you come down. Luigi will look through the hole in the wall. In here is a large golden room full of treasures... and Mario! 'Get me outta heeeere!' Luigi is left speechless by what he sees. Go back to the ladder and catch the Ceiling Surprise to get a key. Go back up to the Courtyard. (Grab the ladder by pressing 
.) Talk to the Toad. He will list off all of the items Mario lost throughout the Mansion: a hat, a glove, a shoe, a letter, and a star. So far, you should have the hat, and the letter. Save your game, and continue through the door to the right of here.
1F: The Rec Room
In this room, the bodybuilding Portrait Ghost, Biff Atlas, is constantly lifting weights. You don't have to catch him, unless you want to fully complete the game. If you want, you can just go over to the treadmill, in the west side of the room, and run on it until the key appears, and then leave by the door to the south, but in either case, make sure you have some Water before you continue.
To catch Biff Atlas, stand by one of the punching bags, face towards him, and press to punch it. Get out of the way quickly, though, or it will rebound and hit you as well. He will get very angry, and attempt to punch you. You need to hit him twice more with the punching bags to 'knock him out', allowing you to catch him. If he escapes, he'll return to his weightlifting, but one more hit will knock him down again. Once he's gone, you'll receive a green chest containing a large amount of cash, and an opportunity to catch another Boo. Leave through the door to the south. Go up the stairs to the right of here. Put out the fire on the door at the landing, and enter it.
2F: The Tea Room
Watch out for any plates that throw themselves at you here. You'll need to catch two Grabbing Ghosts to turn on the power. When you do, a large white chest will appear on the table. To get it, go to the south-east corner of the room, and stand on the circular tile. This changes the gravity of the room, dragging you onto the ceiling. Walk to the left until you find another of these tiles, which will reset the gravity, allowing you to open the chest for the Ice Element Medal. The professor will call you and explain the various uses of the Ice Element. Get some Ice from the bucket on the other table, catch the Boo, and leave. If you check your map (press 
) you'll see several unlocked doors on this floor. For now, go to the smallest room in the north-west.
2F: The Washroom
Catch a Water Element Ghost from the sink, and open the toilet. Inside is a Fiery Gold Ghost. Spray some Water on it, and catch it. A green money chest will appear. Open it, get some Ice from the Tea Room, and leave for the north-westernmost room.
2F: The Bathroom
As soon as you enter, you should notice the silhouette of a woman in the bath. Open the curtain with your Poltergust. Spray some Ice in the bath to stun Miss Petunia, the Portrait Ghost, and catch her. Open the chest for a key. Don't go there yet, though. Instead, go into the room across the corridor.
2F: Nana's Room
Nana is sitting peacefully in her rocking chair, doing her knitting. Check the basket of wool to release the three balls, and they will start rolling around the room. You need to hit Nana with all three balls. Once you have one, she will start to move around the room and shoot 'lasers' at you. If you miss at any point, you'll have to leave the room and start over. Once you hit her with all three, she's ready to be caught. Get the key from the chest that appears, catch the Boo, and head for the last unlocked room on this floor. Along the way, catch a Fire Element Ghost from one of the candles in the corridor.
2F: The Astral Hall
This is an unusual room. If you try to leave by the door to the east, you'll come back in through the door to the west. Light the candles and stand in the centre of the star. The flames on the candles will turn an ominous purple. The doors will be sealed shut. You'll need to fend off two Gold Ghosts, four Shy-Guys, and one Grabbing Ghost to clear the room. Now you can catch the Boo, and leave via the eastern door.
2F: The Observatory
Check out the telescope. Luigi will see the Moon through it. This causes the walls of the room to mostly disappear. Step out onto the small balcony that this event creates. You'll see a meteor shower. You need to catch one of the falling stars, and shoot the Moon with it. Aim to the left of the Moon, and once you hit it, it will explode, creating a glowing path, at the end of which is Mario's Star! Get the Star, leave, and go downstairs to the room where you got the key for earlier, in the Bathroom.
1F: The Billiards Room
The 'legendary' billiards player, Slim Bankshot, walks around his pool table, continuously practising. When he breaks off, you need to catch the billiard balls, and hit him with three of them. Don't worry too much about missing him. Once you've hit him three times, he'll be stunned, and you can catch him. Collect the treasure from the chest that appears, catch the Boo, and go into the next room.
1F: The Projection Room
This room is very similar to the Mirror Room. Turn on the projector, and watch out for the silhouettes of ghosts appearing. This room contains 6 Purple Grabbing Ghosts. When you catch all of them, a red chest containing Mario's Glove will appear. Catch the Boo, and return to the Billiards Room.
Scan the Mirror with the Game Boy Horror (
) to return to the Foyer. Go upstairs to the main corridor of Area 1, and enter the door for which you have the key.
2F: The Twins' Room
Look up at the ceiling to see a mobile made of two toy planes. Point your Poltergust at it, and turn it on. Keep walking around in circles to get it to rotate. After a while, the twins, Henry and Orville, will appear, and challenge you to a game of hide-and-seek. Leave the room to give them a chance to hide. They will call you to tell you they are ready. They are hiding in two of the five boxes. If you don't find them in the first two boxes you choose, you'll need to start over. Check each of the boxes with the Poltergust. The boxes the Twins are hiding in will shake when checked. When you find both of them, they'll accuse you of cheating and attack you, Orville the red twin in an aeroplane, and Henry the blue twin in a toy car. Henry attacks by driving into you, and Orville by bombing you. To defeat them, you must suck up whatever vehicle they are in, then catch them themselves. Defeat Henry first, leaving the floor clear, and making it easier to catch Orville. The last one to be caught is the only one that will drop Pearls. When they are defeated, a red chest will appear, containing Mario's last lost item, the Shoe. With this, you can now return to Madame Clairvoya in the Fortune-Teller's Room.
1F: The Fortune-Teller's Room
Shine your light on the crystal ball to summon Madame Clairvoya. Talk to her. She will start to look at Mario's lost items, talk about them, and make premonitions based on them.
- Mario's Glove: She will recognise you as Mario's brother. She says that he is alive, but cannot yet see where he is hidden.
- Mario's Shoe: Mario has been imprisoned inside a painting above an 'odd' altar.
- Mario's Letter: She reads the letter: "Look out for Boos, Luigi!" She deduces that the letter is warning Luigi of King Boo, who has captured Mario.
- Mario's Star: 'When Boos gather in numbers their power grows stronger!' However, the only thing they fear is the Poltergust 3000.
- Mario's Hat: She sees Bowser, who, she believed, Mario had defeated. She fears that King Boo has somehow revived Bowser.
Once she has seen all five items, the last of her power has been used, and she asks you to catch her and return her to her Portrait. Do so to finally clear this room, and receive a key to a room on the third floor. Catch the Boo, and head up. Along the way, call into the Tea Room to get an Ice Element Ghost. Beware the spiky, explosive ghosts on the second floor, just before the staircase to the third floor.
3F: The Safari Room
Suck at the three deer heads to make three Watery Blue Twirlers appear. Freeze them to catch them. You'll also have to catch two Garbage Can Ghosts. A chest will appear. Get the key inside, and the Boo. Check the box by the chair for a large Heart, save your game, and continue.
3F: The Balcony
As soon as you enter this area, the Boo Radar will start flashing wildly. Head left to find the cause of this: a ring of fifteen Boos. Walk up to them. They all turn to face you, disappear, and reform around Luigi, transporting him to an alternate 'dimension'.
Boss Ghost: Boolossus
The fifteen Boos assume their real form, showing exactly what Madame Clairvoya meant by 'Boos grow in strength in numbers'. Together, they form Boolossus. You must grab him with the Poltergust, and direct him towards one of the unicorn statues. Once he touches it, he will 'pop', and separate into the fifteen Boos, who will fly off in different directions. These Boos are not normal: you cannot catch them simply by sucking them up. Instead, you must get some Ice, and freeze each one individually before you can catch it.
After a while, the remaining Boos will regroup and reform Boolossus. As the number of Boos decreases, they become more difficult to catch. If you just aim at them and fire, they will fly away before the ice beam can reach them. You must face away from them, wait for them to fly towards you, then turn around and shoot at them. When a frozen Boo is caught, it acts similarly to an Elemental Ghost, refuelling your Ice meter. Once you catch all fifteen, Boolossus will be defeated.
You will return to the Balcony, with fifteen more of the total of fifty Boos. A large golden chest will appear. Open it for the Key of Diamonds. Professor E. Gadd will call you, inviting you to dinner at the lab. "Pickled dandelions with barnacles in a diesel marinade"... He will turn the seven Portrait Ghosts of this Area back into paintings. You now have a total of 16 Portrait Ghosts, leaving seven more to be found in Area 4.