The player has to enter the mansion and find the Secret Key at its basement to access the Cinnabar Island Gym. The abandoned building is full of Burglars and Scientists, looting whatever valuable they can find.
The Pokémon Mansion has four different floors. Doors can be unlocked in the mansion by pressing switches hidden in statues scattered around the building. There is a balcony on 3F with two possible points to jump down from, with the eastern one leading back to 2F and the western one leading to 1F and allowing the player to access B1F. There is an exit at the southeast corner of 1F, which transports the player back to the front door of the mansion.
Amongst the rubble and wreckage are journal entries written by the scientist who obtained a Mew and cloned it to create Mewtwo.