Theresia centers on two separate protagonists who are trapped inside a strange, deteriorating facility. Players explore abandoned rooms, piece together a broken past, and work through challenging environmental puzzles in an effort to find a way out.
Presented from a first-person view, the game uses static screens for movement and exploration. Progress depends heavily on inventory management, along with thoughtfully combining items to overcome deadly traps and sealed doors.
Theresia includes an unusual health system in which contact with dangerous surroundings causes injury. As a result, players must watch over their physical condition while also advancing through the game's psychological and environmental challenges.
The game is remembered for its unsettling visual style and for tackling mature psychological themes through a sparse, text-driven narrative. Rather than relying on combat, it builds tension through atmosphere and persistent dread.