“The Grownup”, a 64-page short story by Gillian Flynn, was written in 2014 for an anthology edited by George R. R. Martin; now, it’s gotten published on its own, both as a Kindle single and in a physical copy. For fans of Gone Girl, the chance to get more of Flynn’s writing is a can’t miss opportunity; with only three novels under her belt, and none since the release of that powerhouse, fans (like me) have been waiting for something new. So it’s somewhat disappointing that “The Grownup” is as middling as it is, mainly thanks to a lackluster ending that falls short of the promise before it. “The Grownup” opens well, with a darkly hilarious pair of opening sentences that reminds you how much Flynn loves damaged, horrible people, and just how caustic her prose can be. And for most of the first half, things feel great, with Flynn spinning the story of a young woman who starts life as a con artists, moves on to sex work, and ultimately finds her way into the world of faux psychics, where she meets a desperate woman with all the makings for a perfect – and lucrative – victim. But when she goes to the woman’s house, things take a turn for the unsettling, with a haunted house, a malevolent step-son, and more. Flynn is wearing her influences on her sleeve – the narrator name drops stories like The Haunting of Hill House and The Turn of the Screw – and it’s clear that she’s having fun. But the back half of the story feels incredibly rushed, with an ending that’s less a series of twist and more a series of odd swerves that we can’t ever trust, ending in a way that feels less open-ended and more inconclusive. Worse, though, for all of the nods to great haunted house stories, Flynn never really invests the house with the personality it needs to really be a good scary story. In some ways, that’s not surprising; Flynn lives and dies by her character work and her sharp voice, and neither entirely lend themselves to a supernatural horror tale. But maybe that’s a sign that she’s not really meant to push herself out of what she does so well, because while she’s in her wheelhouse, “The Grownup” is a blast; by the end, though, it all just fizzles out.