Jacob's world is turned upside down when his wife Maja is admitted to a psychiatric hospital with severe depression. Now he is alone with their six-year-old son who reacts by getting an imaginary friend with whom he builds stone portals.

Jacob's world is turned upside down when his wife Maja is admitted to a psychiatric hospital with severe depression. Now he is alone with their six-year-old son, Johannes, pressing deadlines at work and all the worries that come with the hospitalization of his wife. Jacob tries his best to manage the crisis but can't help feeling there is something off in their home especially at night. It doesn't help that Johannes reacts by getting an imaginary friend with whom he builds stone portals. When Maja's depression doesn't improve, but instead reveals a long-term paranoia, and Johannes starts hiding rotten food scraps in his room, Jacob realizes that he must put his own beliefs and sanity on the line if he wants to save his family. Emotional, cinematic and rooted in character Mulm is a psychological horror about the taboos of mental health and how relatives to the mentally ill are affected by it.