The storyline of One Piece Mansion revolves around a successful landlord called Polpo. At the start of the game, Polpo's little sister is kidnapped by the owner of a rival mansion, Chocola. It is the player's job to battle through levels meeting Chocola's objectives and to eventually free Polpo's little sister.
The basic objective of One Piece Mansion is simple. Every resident in the player's mansion has a stress level, which is indicated by a small bar above their head. If this level gets too high, they will eventually move out of the mansion.
Every resident affects the stress levels of the residents around it, in both positive and negative ways. Each resident has a unique stress effect, some having negative to the left, positive to the right, etc. The task as the player is to arrange these so that all the residents are happy. However, if a resident does become stressed out, their effect on other residents changes. For example, sweet Ai-Chan radiates stress-reducing arrows to all rooms around her. However, if she becomes stressed out herself, her effect on other residents weakens.
Polpo, the manager, also has a role to play in the game. By switching to manager mode, he can roam around the mansion and perform two important tasks:
- Blow his whistle to send any Syndicate 5 members on that level back to their rooms.
- Use his fire extinguisher to put out any fires in the mansion.
When the mansion becomes full, the player must build new rooms. If all the current levels are full, the player must also expand the mansion upwards with elevators. Building new rooms may also require demolishing elevators. Both construction and demolition require money.