After being dismissed from her band and abandoned by her boyfriend, aspiring singer Oscar retreats to a secluded cabin in the forest near a deserted town named Tarnation to contemplate her decisions. However, the forest harbors a demonic unicorn, whose infernal master desires Oscar’s blood to finish a ritual intended to summon Satan from the depths of Hell. Oscar is forced to confront a malevolent entity, capable of possessing any soul, in order to not only preserve her own life but also to prevent TARNATION from engulfing the world.