GLOOMY MANOR A-1
Welcome to Gloomy Manor A-1, Poltergust 5000. There's one Boo and two Gem Stones to be found here, but you'll have to come back for the Boo later once you have the Dark Light. Now there's not much you can do without your handy ghost-vacuum, so you should head right for the mansion.
Because you can't get inside yet, so for now, take a peek inside the left-most window, and try the front door anyway. Once you've done both, in either order, a mouse holding a key will appear. Follow him into the garden, then run after him from behind by holding the B button to catch up and snag your prize. Now use it to unlock the front door and reach the Entrance room
Since the northern door is locked, head through the left one instead to reach the garage. Now check the front compartment on the car to find what you came looking for: The Poltergust 5000! This bad boy will allow you to suck up all kinds of things, such as ghosts, treasure, and lots of other things. And you can test it out in this very room by sucking the sheet off a treasure chest in the northwest corner, which contains a key.
To exit the room, you'll need to suck up the debris blocking the door that fell during Luigi's celebration. But before you go through it, you should know that the mission's Boo is also hiding in this room, though you won't be able to expose him until after you find the Dark Light device in Gloomy Manor's 4th mission. Once you have it, return here and shine the Dark Light on the counter closest to the door to reveal a hidden wheel where the Boo's hiding. Now just suck up the spirit balls to force him to show himself.
Regardless, to continue the mission, exit back into the Entrance room. Although you can now unlock the northern door, your path will soon be blocked by yet another locked door, so Instead, use your Poltergust to suck up the door-looking sheet on the right wall to reveal an actual door, which leads into the Mudroom. Inside, you'll spot a key on the ceiling fan if you look up. Use your vacuum on the fan until it spins fast enough to knock the key loose. Now there is another door here, but it requires the Strobulb which you'll find soon enough. So for now, head back to the entrance room and use one of your keys on the northern door.
Make sure to hold the run button as you pass the guards as they'll drop their swords when you get close, which can cause damage and block your path, if you're not fast enough. Now use your second key to unlock the door at the end allowing access to the foyer. Inside, you'll find a pair of ghosts pointing to a chandelier above that contains the Strobulb you need. To reach it, climb the staircase on the right and follow the walkway to the end. Once there, use your vacuum on the fan just above the chandelier until it lowers all the way to the floor. Once it has, hurry back down the stairs to grab the Strobulb before the chandelier returns to the ceiling. This flashlight augmentation allows you to unleash a powerful flash of light that can stun ghosts and activate E. Gadd's doors, among other things. Now I should let you know that there are a couple of gem stones earlier in the mansion you can now collect, but it's actually easier if you hold off until the next mission, which we'll show you in the next segment.
Go ahead and use the Strobulb on the control panel next to the southeastern door in the foyer to allow access to the Coatroom. Interact with the toilet to discover a secret entrance to the other half of a room. Now use your vacuum to tear off the shower curtain to expose a ghost, who'll promptly make a beeline for the room's other half. Use the toilet again to return to the other side. Now if you try to exit the room, the ghost will reappear and block the only way out, and he'll be joined by a few friends. To open the way, simply clear the room of ghosts by first stunning them with the Strobulb, then using the vacuum to suck them up. Well, it looks like there's some ghosts here in the foyer now too; just clear them out like before to complete the mission.