Theresia is a horror / visual novel hybrid which is split into two campaigns; Dear Emile and Dear Martel.

Dear Emile - A tale of twisted love between a mother and her daughter. Snatched from the jaws of death by a woman who reeked of blood, Leanne has grown up in a world of torture, war, and death. Now a young woman, she finds herself in a deadly labyrinth with no memories of her past. Can she uncover the truth behind her imprisonment and escape with her life, or will Leanne be doomed to wander the halls of her prison, haunted by the mystery of Theresia?

Dear Martel - When a fever ravaged his orphanage, a brilliant doctor developed a treatment for the disease, hoping to save the children in his care. Though the fever was defeated, from its corpse rose something far more deadly something far more evil. Will he be able to atone for his mistake, or will his creation spell certain doom for mankind?

Leanne, a teenage girl wakes up in a small, dim room with no memories. The only clue is the word Theresia scribbled upon a piece of paper. The first areas of the game instruct the player on the controls, set the atmosphere and introduce the survival aspect. The building is not only the sub-basement of a larger complex, but also riddled with hidden traps. Most doors are locked, barricaded, or blocked by environmental hazards. Leanne discovers she is in an underground prison facility, which stretches over several underground stories and includes waterworks, storage and torture rooms, and unknown crypts.

In the upper levels, more of the setting and Leanne's background are revealed: the country is losing the war, and the prison is part of a military installation which includes a testing laboratory, run by a woman named Maylee, who Leanne has seen in flashbacks. She also has flashbacks of a young man named Sacha and a silver-haired woman drenched in blood.