She Kills Monsters tells the story of Agnes Evans after the death of her younger teenage sister, Tilly. When Agnes finds Tillys Dungeons & Dragons notebook, she stumbles into the imaginary world that was Tillys refuge. Coming to the realization that she never really knew her sister, Agnes decides to play out Tillys homespun game. Along the way Agnes is forced to fight characters that give her clues as well as questions about her little sisters true identity, all while juggling the expectations of her own reality.