Director Romain Basset's tale follows beautiful young Jessica (Lilly-Fleur Pointeaux) as she returns to her family's countryside estate for her grandmother's funeral. Haunted by recurring nightmares of a horse-headed monster, Jessica attempts to put her studies of "lucid dreaming" to good use, as she semi-consciously navigates through this dream landscape, trying to discover the secrets behind this sinister apparition. But Jessica must also cope with a hostile mother (The Beyond's Catriona MacColl), and the growing realization that the death of her grandmother was actually a suicide triggered by the woman's past traumas and visions. Horsehead is a feverish, ethereal journey through the world of nightmares.