On Cinnabar Island, the Gym is locked, but a note on the door says to find the key in the Pokémon Mansion.

Pokémon Mansion 1F

In the mansion, you can encounter Koffing, Weezing, Rattata, Raticate, Magmar, Ditto, Chansey, and others.

Between two of the columns on the right side of the entry hall, there is a hidden Quick Candy.

There are stairs to the north, but ignore them for the moment. Instead, go northeast, then north, then west. Go to the southwest corner of this room to get a Max Repel.

In this room, check the statue with the glowing eyes to open a door on this floor. Go east, then south from the four columns, to go through the door that you opened, to find a Max Elixir.

Go north to leave this room, then go east, then south. In this area, there is a Max Lure under the table.

Go back toward the entrance, then go up the stairs that are north of the entrance.

Pokémon Mansion 2F

Go south into a room where you can battle Burglar Arnie, who has a level 44 Rapidash. You get 3520 Pokédollars and 3 Poké Balls for winning.

Southwest of the Burglar, there is a Max Ether.

Go north to leave this room, then go east, then go south down a hallway. At the end, go east, then north up another hallway, to find a set of 3 Ultra Balls.

Go south down this hallway, then go west. If there is a locked door there, go north up the hallway, then west to find a statue with glowing eyes. Check on that statue, then go east, and south down the hallway, then go west through the door that you unlocked. In the southeast corner of this room, there is a PP Up. You can read the book on the table if you wish.

Leave this room and go north up the hallway, then go northeast. Then go east, then north into the room. You can read the book on the table. Go east from the table to find a Max Revive.

Go back to the table and go south from there, into the room across the hallway (if the door is closed, go west and check on the statue with glowing eyes, then come back). In the southeast corner of this room, there is an Escape Rope.

Then go west to the stairs, and go up the stairs on the right.

Pokémon Mansion 3F

You can read the book on the table.

Go west from the table and check the ground near the wall to find a hidden Health Candy.

Southeast of the table, you can battle Burglar Simon, who has a level 44 Magmar and a level 44 Muk. You get 3520 Pokédollars and 3 Poké Balls for winning.

South of the Burglar, there is a Rare Candy.

Go north and go down the stairs.

Pokémon Mansion 2F, part 2

Go to the left and check on the statue with the glowing eyes. Then go east, then north, then west into the room. (If the door is closed, go back to the statue and check on it, then come back.)

In the northwest room, you can battle Coach Trainer Rita, who has a level 46 Graveler, a level 46 Dugtrio, and a level 47 Poliwrath. You get 4700 Pokédollars for winning. After the battle, you get TM22 Rock Slide.

Go up the stairs to the right of the Coach Trainer.

Pokémon Mansion 3F, part 2

Go east from the stairs. North of the statue with the glowing eyes, there is a Full Heal.

Go into the room east of the statue with the glowing eyes. (If the door is closed, check on the statue first.) Go east, then north to find TM21 Foul Play.

Then go west and check on the statue, then go south. From there, go east and you can battle Scientist Braydon, who has a level 45 Weezing, a level 45 Magneton, and a level 45 Magmar. You get 2700 Pokédollars for winning.

Go east from the Scientist to find an X Accuracy.

Go west, then south, and go down the stairs.

Pokémon Mansion 2F, part 3

North of the stairs, there is a Hyper Potion.

West of the stairs, you can battle Ace Trainer Louis, who has a level 45 Kingler and a level 46 Primeape. You get 4600 Pokédollars and 5 Ultra Balls for winning.

Go southeast from the Ace Trainer, and go up the stairs.

Pokémon Mansion 3F, part 3

Go west from the stairs, then north, then west past the Scientist, then go south and go down the ladder.

Pokémon Mansion 1F, part 2

Go north from the ladder to find a set of 5 Silver Razz Berries.

Go south from there to battle Scientist Ted, who has a level 45 Electrode and a level 45 Muk. You get 2700 Pokédollars for winning.

Go south from the Scientist, past all the plants, to find a Fire Stone.

Northeast of that, there are some stairs. There is also a teleporter to the east that will take you outside if you want to leave for now. To get back here from the entrance, go north and up the stairs, then go to the northwest room and go up those stairs, then go southeast and go down the ladder.

Go down the stairs in the room with the wilted plants.

Pokémon Mansion, B1F

Go northwest from the stairs, then go west and you can battle Burglar Lewis who has three level 44 Koffings and a level 44 Weezing. You get 3520 Pokédollars and 3 Poké Balls for winning.

Go south into the small room (if the north door is locked, go in from the left side). You can read the book on the table. South of the table, there is TM52 Sludge Bomb.

If the north door is open, check on the statue, then go out through the door on the left. From there, go west, and check in front of the rightmost bookshelf to find a hidden Smart Candy.

Go west to find a Max Potion.

From here, go east, then north, then east past the room, then north. Keep going north and you can battle Scientist Ivan, who has a level 45 Electabuzz. You get 2700 Pokédollars for winning.

Go north and check on the beds to heal up your pokémon.

Check the lamp on the right to find a hidden Courage Candy.

Go to the left and check the statue with the glowing eyes to flip the switch.

Go south, then go west from the Scientist. Check the lamp west of the Scientist to find a hidden Tough Candy.

Go west, then north into the room, to find a Rare Candy.

Check the pile of books to the left to find a hidden Tough Candy.

Go south to leave this room, then go west into the next room (if the door is closed, go back to the beds in the northeast and check the statue there, then come back here).

Go northwest to find a Max Elixir.

Go south to the next room to find a Secret Key.

You have found everything that there is to find here, so you can either use an Escape Rope to leave, or go back into the room with the beds in the northeast corner of this floor, and check on the statue, then go south and go up the stairs. Then you can go east and step on the teleporter to leave the mansion.

Now that you have the Secret Key, you can enter the Cinnabar Island Gym.