The key to the Gym is lying around somewhere in the decrepit building. Also in the mansion are a handful of journal entries that describe the secret experiment that was conducted on the island. One of the obstacles between you and the Secret Key are the Pokémon statues scattered around. Each one has a hidden switch inside, which opens some doors and closes others when pressed. A handful of Burglars also prowl the wreckage for abandoned items, so be on guard.
Head to the northwest room and flip the switch inside the statue. Then backtrack and climb the stairs straight ahead of the main entrance.
Flip the switch inside the nearby statue and head to the northeast, where the first journal entry is lying on a table. Beyond that is a Calcium near the east wall. Climb the stairs in the northwest room.
Head east through the north door to find an Iron. Flip the statue switch to open the south door, then jump off the smaller balcony to the right.
Go west to find a second journal entry in the southwest room. Take the stairs near the statue to a separate area of the third floor.
This area of the third floor holds a Burglar, a third journal entry, and a Max Potion. Be careful not to fight the Burglar while on the right side of the table, else you will become trapped in the corner. If trapped, use Dig or an Escape Rope, or allow the wild Pokémon to cause you to black out.
Flip the switch and climb the stairs in the northwest room.
Approach the Scientist again, and jump off the larger balcony to the left to land on the first floor.
Battle the Scientist, grab the Carbos, and take the stairs to the basement.
Fight the Burglar in the southern room, and pick up the TM14 (Blizzard) near the statue. The final journal entry is also located here. Grab the Full Restore near the floor's southwest corner, and flip the statue switch. Follow the narrow hallway in the northeast to battle one last Scientist, and pick up a Rare Candy and TM22 (SolarBeam) on the way westward. The Secret Key is located in the final room.
CINNABAR ISLAND POKéMON GYM LEADER: BLAINE
The Hot-Headed Quiz Master!
The Cinnabar Gym specializes in Fire-type Pokémon, so Water-, Ground-, and Rock-type moves will deal the most damage. Avoid bringing Bug-, Grass-, and Ice-type Pokémon—unless those Ice types are alsopart-Water. Play with fire and you are bound to get burned, so be prepared with a few Burn Heals just in case. Blaine enjoys riddles and puzzles, and uses this to test his challengers. Answering each question correctly unlocks the next door, allowing you to bypass each Gym Trainer.
With the Cinnabar Gym defeated, surf north from the pier to reach Route 21.
The waterbound Route 21 is a quiet route that connects Cinnabar Island to Pallet Town. Before continuing on to the final Gym, there are some missing items and Pokémon that you can get, and there is another legendary Pokémon to find. To begin, fly to Vermilion.