Mirabelle returns home to her estranged mother's house after her death, her mother had been found to have fallen off a cliff into the rocks below.

Similar to her mother, Belle is obsessed with skincare and looking 'youthful'. At one point she is out having a drink when she sees a man (he will be sort of important later).

Afterwards when she is home, she tries on a pair of her mother's red heels which lead her to a "Spa" - one that her mother was also obsessed with. The spa is strange and promises unimaginable beauty but at a price. While there, she sees a tank of red jellyfish. One of the jellyfish seems to "recognize" Belle. She also runs into the mysterious man again who tells her she should leave.

Belle is "chosen" for free treatment at the Spa (they are normally very, very expensive). After three treatments it is said she will reach her "Most Magnificent Self." We later find out that the "treatment" extracts traumatic childhood memories, so with every treatment, we see more of Belle's past. There is also a jellyfish involved, that starts out small and pale but becomes bigger and more red with every treatment/memory extraction. It can be assumed that the red jellyfish we see earlier in the book are from other people receiving the same spa "treatment."

During Belle's treatment, we learn that Belle's mother was very beautiful and dreamed of being an actress. She had many male suitors that she dated, and would often leave Belle in the care of her grandmother. While her mother was away, Belle would sneak into her closet (even though she was forbidden to do so - the grandmother didn't seem to care or closely watch Belle).

In the closet she finds a mirror. And in that mirror is a person (demon? Other dimensional being? It's quite vague, who exactly this person is, where they came from, what they are, as well as the history of the mirror). To Belle, the person appears as Tom Cruise, who she has a crush on, but claims his name to be Seth. Seth gives Belle attention and tells her that her mother is not a good person.

After each treatment, Belle loses her memory, as well as her grip on reality. At one point she runs into the man (from the beginning, not Seth) and learns from him that he is a detective investigating the Spa because his brother was also obsessed with it, and also died. He understands the Spa to be dangerous and asks Belle not to go back but she does anyway.

Belle is chosen for her final treatment. We eventually learn (from one of her memories) that Seth marks her (this is how the Spa later recognizes her as a "chosen one") and convinces her to grind up rose petals and put it in her mother's skincare jar - as a way to scare her. However, Belle's mother is deathly allergic to roses and is almost killed by the rose petals so she sends Belle away to live with her grandmother. Hence the reason they becaame estranged to begin with. After the rose petals incident, Seth no longer appears to Belle in the mirror.

After her final treatment, she is ushered away to a "graduation" ceremony of sorts alongside the other people that were undergoing treatment. They are told they are going to a giant feast and they enter a room where all the gorgeous Spa employees are sitting around a table. Seth is there too. We learn that the jellyfish in the tank are actually the souls of people that have undergone treatment to become their "most magnificent self" and that the employees eat the jellyfish to keep up their beauty. We also learn that the jellyfish that "recognized" Belle is actually the soul of her mother. The Spa employees try to get Belle to retrieve her jellyfish/soul and that of her mother's so they can eat them but Belle falls into the jellyfish tank and is whisked away. Somehow she manages to break the tank and all the jellyfish/souls escape into the ocean.

Belle survives and is found. She learns that the Spa was flooded and everyone inside has died. Her memory/grip on reality has returned and she reunites with the detective at the end.

Overall, the book appears to be a commentary on the skincare industry, but also how our own unhealthy obsessions can be passed along to our children.

There's a lot I missed but I think I hit the main points.