In "Mystery Island: House Rules," the successful Mystery Island team envisions expanding their venture and decides to collaborate with the Mystery Mountain crew for a lighthearted murder mystery game. However, the atmosphere quickly shifts from playful to perilous when an actual murder occurs right before their eyes, shattering the camaraderie and plunging everyone into a real-life investigation. As the lines between the game and reality blur, the teams must unravel the truth behind the shocking crime before it claims more victims.