Gemma, the heroine of the debut novel from Barbara Radecki, is 16 years old, and lives with her father, Jonah, on a small island off the coast of New Brunswick. Gemma’s mother decamped when the girl was just a baby, so it’s been just her and her dad all along, though the other island residents make for a warm – and unsurprisingly quirky – extended family. It’s a quiet life: Jonah, who in addition to his role as lighthouse keeper also captains the local ferry, keeps busy with his science experiments in the locked shed out back, and Gemma spends her days alone save for her imaginary friend Addie, who sleeps in the closet.

Their routine is shattered when a mysterious woman arrives on the island. She’s obviously not who she is pretending to be, and it’s too early in the season for her to be a tourist, so who is she? What is she hiding? These mysteries only deepen when the woman is drawn into Jonah and Gemma’s life, forcing Gemma to take a closer look at what is going on around her, and what happened to her in the past.

All the elements are in place for a winning YA psychological thriller, and for the most part, The Darkhouse succeeds. The novel is well paced, with a narrative just twisty enough to keep even seasoned readers guessing. Radecki, a screenwriter and actress, writes with a clear prose style and good insight into the often-contradictory nooks and crannies of Gemma’s troubled psyche.

The novel stumbles somewhat, however, when Gemma runs away from home and the island. It’s not just that the developments in this stretch are a bit too convenient (not even Dickens had a gang of street kids as welcoming as the ones Gemma encounters, and her interaction with two brothers on the road is positively sweet), but the shift away from the island’s cloistered, uncanny atmosphere allows the pressure to ease, robbing the narrative of much of its tension. Thankfully, Radecki reverses course in a commanding manner by the novel’s close.