Schoolteacher Tessa Ross and her husband, police detective Brady Ross, return to Tessa's family's home where they attend a meeting led by her sister Alison Hawthorne-Price's campaign manager Naomi, prepping them all for interviews. Also in attendance at the family home are Tessa's brother Cam Hawthorne, a cartoon artist and ex-drug addict, Cam's son Jack Hawthorne, politician Alison, Alison's husband Tom Price and twin daughters Harper and Violet, and the Hawthorne parents, Madeline and Mitchell. Madeline has removed pictures of their fourth child, Garrett, from their walls in anticipation of the pre-interview.

Tessa and Brady are in the midst of trying to conceive. Alison is preoccupied with worries about defeating the incumbent mayor in the upcoming election.

Alison gets a phone call informing her that a tunnel, constructed with Hawthorne concrete, collapsed, representing a potential issue for her in the election. It turns out that the fault was in the bolt (not the concrete), making it not an issue for the Hawthorne family. The police find a belt in the concrete, linked to a 2002 unsolved murder and to a serial-killer case that went cold fourteen years earlier -- the Silver Bells Killer, who left no evidence behind at the scene of each of his killings, save a small silver bell. Brady is brought in on the case.

Alison gives a press conference, intending to use the discovery of the belt to insinuate that she will solve the cold case where the sitting mayor could not. Mid-conference, Mitchell collapses, suffering from a massive heart attack.

Mitchell survives but is in bad shape. The siblings and their mother discuss Garrett, the fourth Hawthorne sibling who left them all behind fourteen years earlier. Believing he'd want to visit their father, Tessa secretly calls the number she has for her brother, leaving a message with a man who answers. Cam's ex and Jack's mother Sophie, still currently a drug user, arrives at the hospital to support him but Cam refuses her.

While staying at the house to support her mother, Tessa spots her brother Cam in the shed and confronts him, realizing he's searching for drugs that he stashed. She insists on helping him look for it so they can throw them out and prevent Cam relapsing. While searching, she stumbled upon a box -- inside is a newspaper clipping about the Silver Bells Killer and a few small silver bells of the same type the killer left.

Cam initially suggests that the box could have belonged to the previous home owner, but the article has one of his cartoon doodles on the back, proving it belonged to the family. Cam and Tessa panic, believing that this might mean their father is the SBK. 

The next morning, Jack shows his dad the gruesome drawings he's done of deceased people. Cam is unnerved and tries to encourage Jack to draw fantasy cartoons instead, but Jack is uninterested.

Tessa answers the door to find a distraught older woman looking for her missing cat, Caramel. When the woman leaves, Tessa spots a disheveled, bearded man who she initially doesn't recognize. He greets her as "little sister" -- it's Garrett. They warmly reunite and Garrett drives Tessa and Cam to the hospital to see their father. Garrett explains that he'd been living in Maine, 

As Garrett reunites with his father, Mitchell suffers another heart attack, slipping into a vegetative state. Back at the house, the family discusses Mitchell's predicament, and the cat lady returns, forcing missing cat flyers into Madeline's hands. Garrett reluctantly agrees to stay in his old room, which has a closet still full of his old athletic gear and awards from high school. Jack introduces himself to Garrett and they bond over their shared morbidity and "weirdness."

The next day, Garrett sharpens his hunting knife on a rock and shaves himself with it in the family kitchen, alarming Cam, who comes upon him doing that.

Cam and Tessa do research on the SBK. Cam points out that the SBK murders stopped fourteen years ago -- when Garrett left. He believes Garrett could be the killer but Tessa can't believe that. They bring Alison in on it. Alison convinces them not to say anything until after the election, insisting that if it got out it could destroy all of their lives.

Cam is tempted to do the drugs that he found. Suddenly, he hears a cat screeching and runs into the basement where he finds Jack, who has cut the tail off of Caramel, the neighbor lady's cat. Cam rushes the cat and the tail to the vet, with Jack, who cannot understand what he did wrong. Cam asks Tessa to watch Jack while he goes to the hospital to see their father. Instead, he goes to see Sophie. They do drugs together and have sex. He confides that he believes there is something wrong with their son.

Alison's daughters tell her that they heard Garrett whisper to Mitchell, "I'm gonna tell them it was you" at the hospital, right before Mitchell had his second heart attack.

Tessa tucks Jack in after he's fallen asleep and finds his drawing of a dissected cat.

Unable to sleep, Alison slips out of bed and texts her campaign manager Naomi that she can't sleep.

At the hospital, Madeline knits as she watches over Mitchell. He awakens from his coma and tries to insist that they must tell the truth. Madeline tries to calm him down. She removes his vitals monitor and stops his flow of oxygen, killing him.