Theresia.. is a horror adventure game. The game incorporates two distinct types of gameplay: three dimensional first person segments and two dimensional point & click adventure segments. The player has to find and combine items and avoid traps to survive.

The game has two parts; The first puts the player in the role of Leanne, a young girl who wakes up in a dark, abandoned prison with no memory how she got here. The player must guide Leanne through the prison while uncovering items that help her regain her lost memory and eventually to escape the prison. The second part puts the player in control of an unnamed adult man, who must escape an orphanage where a brilliant doctor had developed a treatment to a fever ravaging the orphanage, which turned out to become a bigger threat than the fever itself.