Allie Kim’s best friend is dead, and she knows who killed her. The only problem? She doesn’t have proof. But the killer knows that Allie knows, and he’s going to make her life hell if she so much as thinks about telling anyone.

Jacquelyn Mitchard’s WHAT WE LOST IN THE DARK picks up Allie’s story directly after WHAT WE SAW AT NIGHT ended. While the first installment in the series felt like a mystery-thriller, its sequel feels like a horror-thriller. This time around, we know who the bad guy is: Garrett Tabor, the charming young ski instructor and doctor specializing in XP, the medical condition from which Allie suffers. XP stands for xeroderma pigmentosum. It is a genetic disorder that gives a person an allergic reaction to sunlight. The small Minnesota town in which Allie lives boasts one of the top XP clinics in the country, which is why she, her boyfriend, Rob, and her late best friend, Juliet, have lived there together since early childhood. Since the XP clinic is a point of pride for the town, it won’t be easy for Allie to convince the authorities that Garrett Tabor is anything but an upstanding citizen.

According to official reports, Juliet killed herself by jumping off of a bridge. But Allie and Rob have serious doubts that this is the full story, even though Garrett Tabor’s predatory abuses as her ski instructor did weigh on her. Allie knows that Garrett Tabor had an even more direct hand in her disappearance. She received panicked voicemails from Juliet after her mangled body had been discovered in the river, and this adds another layer to the cruelty of Garrett Tabor; it seems that he sent the voicemails to Allie to taunt her.

"Mitchard excels at building tension, but she also constructs painfully poignant relationships between friends and family members. This combination makes WHAT WE LOST IN THE DARK feel horrifying, personal and ultimately satisfying."

Before Juliet died, she, Allie, and Rob used to fill their nights with Parkour. Since their disease bars them from interacting with the outside world in the daytime, they could at least dominate the town at night. By knowing every inch of the buildings they scaled, they could exert some control over a place that usually controlled them. But now that Juliet is gone, Allie can’t practice Parkour without remembering her late friend. When Rob suggests an alternative, Allie is wary, but intrigued.

Rob has developed an interest in free diving. This means submerging oneself as far as possible below the surface of a body of water before returning to the surface. It requires intensive breath control, and those who practice this risky activity run the risk of losing consciousness in the depths. But Rob’s and Allie’s life spans are already severely limited by their disease, and this gives them a thirst for adrenaline rushes. They want to live while they can.

Although free diving enraptures Rob, Allie is a bit more hesitant. She fears the dark water; she can never be sure exactly where she is diving, even with a headlamp. Parkour was all about knowing intimately the details of your surroundings and turning them to your advantage. Free diving is about throwing oneself into the unknown. Breath is the only thing a free diver can really control, and the isolating claustrophobia of remaining underwater for long stretches of time makes Allie nervous. Still, she wants to do it for Rob. He enlists the help of Wesley, the salt-of-the-earth outdoorsman who is a free diving enthusiast himself. Wesley trains Allie and Rob to control their breath and supervises their dives. But during their first real dive, Allie stumbles across something horrific that further incriminates Garrett Tabor, if only she could show the authorities and tie it irrevocably to him.

While WHAT WE SAW AT  NIGHT followed Allie piecing together the various strands of Garrett Tabor’s guilt, WHAT WE LOST IN THE DARK focuses more on her desire to report him and her fear that such an action would have dire consequences for her family and friends. Garrett Tabor texts her photos of her mother and younger sister, alone and vulnerable, with the implication that he could abduct either of them at a moment’s notice. Allie interns at the medical examiner’s office, which means that Garrett is always looming over her with quiet threats. The tension soars to nail-biting heights, and the reader bristles with the injustice that Garrett Tabor is still free.

The heart of this story lies in the relationships that Allie has with Rob, with her mother, Jackie, and with her little sister, Angela. The mutual affection that Rob and Allie have for each other is obvious. They are the only two who know the truth about Garrett Tabor, and they are each other's only friend with XP. But their relationship goes beyond mere convenience; it is built on years and years of childhood friendship, and this closeness has lasted and developed as they grow up. Still, their relationship is not without strains, and this book allows us to see the result of those pressures.

Jackie worries constantly about her daughter and gives her everything she can, including the freedom to be out at night. Allie, scared of dying sooner than she should, pushes her mother away sometimes. Jackie’s heartbreak at these moments is palpable and wrenching. Nine-year-old Angela is old enough to be scared about losing her sister prematurely to her disease, but not yet old enough to be tactful in how she expresses this. Angela tells Allie that her friend at school said that Allie is going to die soon. Allie, scared and hurt, has to explain the implications of her disease to her sister again and again, because she knows that Angela’s insensitivity comes from her acute fear of losing her big sister.

Mitchard excels at building tension, but she also constructs painfully poignant relationships between friends and family members. This combination makes WHAT WE LOST IN THE DARK feel horrifying, personal and ultimately satisfying.