Siyama is the story of three Thai youths who are transported back in time to an ancient village in the last period of Ayutthaya kingdom. They find themselves in the midst of a vicious war with Burma before Ayutthaya is burned down (in April 1767).
A group of friends from Bangkok decide to visit an abandoned house inhabited by a spirit who has the power to grant wishes, if promised the appropriate offering. With their graduation getting closer and closer, most of them pray for decent grades, but one of them, a skeptical girl called Moss, makes a mocking pledge that she will do anything in return for the satisfaction of her friend's desires. while their friends are careful to make sure they provide the promised offerings as their wishes are fulfilled, Muay, Pin, and Moss are not so diligent. Before long all three are subjected to a series of bloody manifestations, courtesy of an angry and vengeful spirit determined to collect what is owed.
Ratri fancies the husband of her best friend, but he's not interested in her. She enlist the help of various witch doctors and medicine men to concoct her numerous of ancient Thai folklore recipes of love potions. Some recipes also require her to perform various sex acts in order for the magic to work. Each potion seems to work slightly different from one another and there are side effects which accumulate with dire consequences.