Fed up with the hell of being 16 forever, like some perpetual Twilight target demographic, world-weary Eleanor (Saoirse Ronan) submits a baroquely revealing memoir to her very concerned teacher (Tom Hollander). “It’s like Edgar Allan Poe and Mary Shelley got together and had a very strange little child,” the teacher tells a colleague, not wholly disapproving but clearly dismayed. Neil Jordan’s downbeat and darkly sudsy gothic melodrama, adapted by Moira Buffini from her own play, seems similarly mashed-up, and maybe the only way to know if it’s improbably brilliant or highly unnatural is just to invite it in and let it consume you. Yes, Eleanor is a vampire, like her variously bedeviled mom, Clara (Gemma Arterton), and underneath all the genre appliqué, theirs is basically a story of attractive young women as eternally dispossessed transients. Here, Ronan’s striking eyes have the perfect 200-year stare, and Arterton’s sensuality seems downright supernatural for a pointed reason. With convenient help from a wimpy, unwitting accomplice (Daniel Mays), the ladies take up in a forlorn seaside resort town, whose ashen solemnity Jordan uses to suggest the vampire movie as undead tourist attraction. Staging gory shocks and ironic japes with equal casualness, he also flings out scraps of context: Eleanor’s budding connection with a sickly classmate (Caleb Landry Jones), Clara’s fateful run-ins with veterans of the Napolenoic Wars (Sam Riley, Jonny Lee Miller), and everyone’s visits to a gloomy island whose waterfalls run red. One certain conclusion: immortality bites.