Concept: An X-Files-esque conspiracy thriller that’s basically two completely different games depending on which agent you’re playing as at the time.
When you’re playing as the Down to Earth agent, it’s a straight-laced police procedural adventure game – spot the clues, interrogate the witnesses, etc. Realistic graphics, subdued jazzy soundtrack, psychologically plausible personalities and motivations, and so forth.
When you’re playing as the True Believer agent, however, the game goes full gonzo. Many NPCs who appear human from the Down to Earth agent’s perspective appear as various supernatural beasties (werewolves, mothmen, etc.) and have completely different dialogue trees. Some areas become inaccessible because they’re barred by hazards that only the True Believer can perceive, while new areas open up thanks to doors and passages that don’t exist for the Down to Earth agent. The graphics and soundtrack become more stylised, more colourful, and more grotesque.
You can switch viewpoints at any time, unless you’re in an area where only one agent is present. It’s never made 100% clear which agent’s perspective is “real”, if indeed either of them are, and the gameplay actively obfuscates the truth; one puzzle’s solution might rely on the True Believer sneaking into a building using an entrance that only exists for her, then letting her counterpart in, for example, while another might depend on the Down to Earth agent’s ability to simply walk through an obstacle that only the True Believer can see. Items taken from one version of reality can freely be used in the other.
In the end, the twist is that there’s no twist. Neither agent’s perspective is proven wrong, and they go on with their lives, each thinking the other is crazy.