Libby Jones, adopted as a baby, knows little about her origins. But she has known for most of her life that they are dead, that there is a will, and that she’ll be contacted by the trustees of her parents’ estate when she turns 25. That day is finally here.

Opening the letter upends Libby’s life. She has inherited a mansion in the posh neighborhood of Chelsea in London. And she learns she has a brother and a sister — or, had.

What seems like a stroke of good fortune, transforming the financially struggling Libby into an instant millionaire, becomes a dark tunnel to her past life. We learn that three adults died in a suicide pact, and that the two teen siblings were never seen again. Needing an ally in her quest for answers, she contacts the London journalist who, 25 years ago, chronicled the macabre story of the dead bodies and the crying baby in the mansion.

As Libby begins her journey, several parallel characters quietly surface to tell their own stories of growing up there. The reader knows these paths are on a collision course.

Flashbacks help fill in the events that take place inside the walls of that mansion as a once-great family descends into hell. Their home becomes a fortress of psychological manipulation, of child abuse, of a cultlike culture led by a man with an engulfing ego and a fearsome rage.

Lisa Jewell’s story is spellbinding, somewhat Gothic despite its contemporary setting, and impossible to put down. By the time we unravel the last of her mysteries, we’re almost choking on the malevolent threads that weave themselves into a coy and satisfying conclusion.

“The Family Upstairs” By Lisa Jewell. (Atria Books, 340 pages, $27.)