Mientras duermes

César, a concierge of an apartment building in Barcelona, is on a mission to make his tenants miserable. He sets his sights on Clara, but soon finds that she is harder to upset than he expected. As Clara's life becomes increasingly complicated, César resorts to more sinister methods to achieve his goal, including the use of chloroform and manipulation. Clara's boyfriend Marcos becomes involved, leading to a twisted and deadly confrontation between César and the pair. Meanwhile, César's own dark past and loneliness are slowly revealed, adding to the film's sense of tension and unease.