Only a few people possess the unique ability to read people's hearts; those people are called psychic analysts, and Katsuya Furuyagi is one of them. One day, a fellow analyst pays him an unexpected visit and directs him towards a mysterious old man named Kamiya. Reading the old man's mind, Furuyagi ventures into the realm of memories, where a woman's ghost is attempting to avenge her own murder.

Aya is the third installment of the Psychic Detective series. Like in the previous entries, it is a Japanese-style adventure game with first-person view and menu-based interaction and navigation. Commands such as "Look", "Ask" and "Move" must be used on most screens in order to advance the plot. Talking to characters and contemplating about the case play an important role.