Lily was abducted when she was sixteen and has been held captive for eight years in a cabin basement. During this time, she has given birth to a daughter. One fateful night, Lily realizes that he has forgotten to lock the door, or is this another test? Lily wakes her daughter and flees into the night. Overton's story is what happens next.
The book has no real surprises or plot twists; there is nothing shocking, are no revelations, and the writing is average at best. She could have done much more with the characters—especially Lily given that she was held captive for so long and experienced horrific abuse and neglect. She would need intense therapy to overcome the incredible trauma endured, but that is not the angle that Overton took with Lily's story, even though it mostly takes place after she returns home.
The opening few chapters are the best in the book. There was definitely a hook, and my criticisms aside, there's some good writing in these beginning chapters.
Please stop trying to hook people by mentioning Gone Girl and Girl on the Train when marketing your book. If anything, this is closer to Room, but again, you should just not do these comparisons when a book is sub par to all of the books mentioned in this paragraph.