The following version of this book was used to create the guide: Jewell, Lisa. The Family Upstairs. Atria Books, 2019.

Jewell divides The Family Upstairs into a prologue plus four parts: Part I (Chapter 1 through Chapter 33), Part II (Chapter 34 through Chapter 48), Part II (Chapter 49 through Chapter 65), and Part IV (Chapter 66 through Chapter 69). While the chapters set in the third-person perspectives of Libby Jones and Lucy Lamb are in the present tense, the majority of those narrated in first-person by Henry Lamb, Jr., are set in the past tense as he recounts events that happened between 1988 and 1994. Each of the four parts contains their own rising climax and big reveal, leading up to the convergence of these three characters in the end of the book.

The novel begins with a brief, half-page prologue narrated by Henry (although the narrator's identity is not divulged until later). Henry tells the reader that his childhood went from somewhat abnormal to becoming extremely dark. Things changed when a baby came into the picture, at which time Henry was 16 and his sister was 14.

Libby Jones is then introduced as one of the three main protagonists. She has recently turned 25 and stands to inherit a large house on Cheyne Walk in Chelsea, London, England. Libby is riddled with disbelief at first, and her whole life is turned upside-down. Meanwhile, in France, the second main protagonist, Lucy Lamb, is homeless with her two children (Marco and Stella) and their dog, Fitz. Stella's father was the love of Lucy's life but abandoned them to return to his home country. Lucy gets a reminder on her phone that the baby is 25 now, which immediately connects the two women for the reader. The third main protagonist (who also serves as something of an antagonist as the novel progresses) is Henry, who relates to the reader how his life changed for the worse in the 1980s. He and his father (Henry Lamb, Sr.), mother (Martina), and sister (who is revealed at the end of the novel to be Lucy) lived in the posh Cheyne Walk house that Libby is currently set to inherit. Martina was a beautiful socialite, and Henry, Sr., inherited a sizable fortune from his father. The house was lavish, but the items were merely a way for the Lambs to cover up their unhappiness.

Problems developed for the family when a woman named Birdie came with her band to film a music video inside the house. They were only supposed to stay for a few days, but it turned into a more permanent living situation. Through Libby's current-day perspective, it is revealed in an article written by Miller Roe that Henry, Sr., Martina, and another man were found deceased inside the home in 1994 as the result of what looked like a cult-based suicide pact. By that point, Henry, Sr., was wheelchair-bound. The Lambs' two teenage children and several others were unaccounted for. As far as anyone knew, the Lambs had sold their luxurious items long ago and slept on mattresses on the floor. They even made their own clothing. However, Libby (whose birth name is Serenity) was left in a stylish old crib from Harrod's with a lucky rabbit's foot as her only possession. A suicide note left with the adults gave her name and asked that she be given to nice people.

Meanwhile, Lucy is desperate to return to London. She reluctantly goes to her abusive ex-husband, Michael, and asks for his help obtaining fake passports for her, the children, and even Fitz. Michael agrees under the pretense that he will get to have sexual intercourse with Lucy. Lucy mentally prepares herself for what will happen despite her recollections of Michael's past abuse. Michael produces the passports and has Lucy meet him at his home for dinner. Michael and Lucy begin to get intimate, but a glass breaks and jabs Lucy in the back, causing her to scream in pain. She tells Michael to stop, but he will not. Lucy grabs a knife and stabs Michael. He bleeds to death, and she quickly plans on how to hide the murder. She cleans up, takes the passports, and leaves.

Henry then retraces the narrative back to the time when David Thomsen and his family entered their lives. Birdie invited David in as an alternative healer for Henry, Sr., whose health was steadily declining. At dinner the first night, Henry realized that David's intentions were negative as David slowly began to seduce Martina. David's power would grow stronger as Henry, Sr.'s, weakened. Henry's father told him that David made Martina feel like her life had meaning. However, as time went on, the Lambs gave up their luxury items and services. Martina thought it made them free, but young Henry felt like his life was starting to fall apart.

Libby meets with Miller, and the they, along with Libby's friend from work, Dido, go to the Cheyne Walk house. Miller reveals that he has only found out a little bit about David, who was the third corpse found alongside the Lambs. He, his wife Sally, and children (Phineas and Clemency) came from France in September of 1988. The trio end up finding a man's sock in the house, and it is clearly a new one, suggesting someone has been there. Libby decides to stay the night in the house with Miller, and they discover that someone wrote "I AM PHIN" throughout the house. They then see a man entering the house. They confront him, and he introduces himself as Phin.

Henry then tells the reader about his burgeoning romantic feelings for Phin. One day, he and Phin secretly left the house, and Phin bought LSD with money he stole from David. Phin confided in Henry that David took a family heirloom from the people they had stayed with in France, and that was part of why they were made to leave. Phin warned Henry that David was going to take everything from his family and break every person down. They snuck up to the house's rooftop landing and did some of the LSD. Henry felt like he and Phin melded together, but their pleasant high was ruined by Birdie and David, who discovered that the boys were high. Phin revealed that Henry told him he had seen David and Birdie kissing. This prompted Sally to move out.

Henry started to bond with Birdie's boyfriend Justin, who practiced witchcraft and grew herbs in the garden. Henry became Justin's apprentice. At that time, David began enforcing a number of ridiculous rules that were mostly put in place because of Phin's rebellions. David even got physically abusive with Phin, who escaped to live with Sally, but she brought him back. One night, he and Henry went out toward the Thames, and Phin pushed Henry in after he got angry over Henry staring at him. Henry told the adults what happened, and Phin was locked in his room for a week by David. None of that sat well with Justin, who confided in Henry that he wanted out before he finally fled. Justin left the herb garden and his books on witchcraft to Henry, who secretly stashed them in his room.

Back in the current plot, 'Phin' locks Miller and Libby in a bedroom in his Airbnb flat after getting them drunk and telling them some of what happened in the Cheyne Walk house. Miller and Libby think it is odd that 'Phin' locked them in, which he claims was an accident. He then shows Libby and Miller the video that Birdie's band filmed inside the house. After they have left the flat, Miller and Libby agree that 'Phin' drugged them during the night, took their phones, and went through them.

While this is happening, Lucy and her family depart from France and make their way to England without being caught with the fake passports. They make it to the Cheyne Walk house and enter through the back door. In her mind, Lucy relives some of the traumatic moments that happened inside the house. Still, she and the children wait there for the baby to return.

Then, Henry recounts how Martina became pregnant by David. Henry was livid with David and ashamed of his mother, but Martina thought of it as a happy occasion. Henry realized that Martina was being used to produce an heir to the Lamb fortune so that David could legally gain access to their estate. Henry devised a plan to grow the right herbs and use them to cause his mother to miscarry the baby. The baby died and was buried, but David's attention then turned to Lucy. Martina never quite recovered from the loss of the baby and withdrew from everyone in the house. When he was punished by David, Henry decided that he was going to get rid of David for good. After Henry was released from his room, David announced that Lucy was pregnant with his child. That was when Henry realized that David did not just want the Lambs' money, he wanted the house too. Henry broke into David and Birdie's bedroom, found his father's will, and also took some money and his old pencil case that he used to carry to school.

In the present, Miller shows Libby all the details he has gathered on Birdie. Birdie has not been heard from since 1994, and her well-to-do Catholic family did not seem to care about finding her. Justin has also been off-the-grid for several decades. Sally, however, is working as a life therapist in Penreath, Cornwall. They get the address and go to speak with Sally. They speak with her and try to gather from her where Clemency is, but Sally refuses to tell them. After they leave, Libby confides in Miller that she is as organized and rigid as she is because of how chaotic life was with her adopted mother.

Lucy and the children are surprised by a man who enters the house. It takes her some time to realize that this man is Henry. Lucy introduces Henry to her children, and Henry admits to having met Libby, thereby outing himself as 'Phin'. He is also using a device to track Libby's phone and shows Lucy that Libby is currently with Clemency.

After they leave, Libby confides in Miller that she is as organized and rigid as she is because of how chaotic life was with her adopted mother. They meet with Clemency, who reveals that Henry had something of an evil streak in him, as he poisoned Birdie's cat and cut off her tail. Henry immediately interjects with his narrative, giving the reader his version of events. He admits to accidentally killing the cat and cutting off her tail. He also corrects Clemency's claim about him pushing Phin into the Thames. Henry claims they were pushing each other because Henry tried to kiss Phin, who rejected him. He then recounts when Lucy gave birth to baby Serenity.

Clemency asks Miller for some privacy and tells Libby the rest of the story. She agreed to help Henry with his plan to rescue the baby. He was going to knock the adults out with an herbal potion. The potion worked, and the teens went into David and Birdie's room to get supplies and money. However, the potion did not fully work on Birdie, and when she lunged at Lucy to try to grab Serenity, Henry killed her with a tusk or antlers. Henry checked the other adults and realized they were dead.

Henry's narrative cuts in and explains how he buried Birdie and fled with Lucy, Clemency, and Serenity. They left Phin behind because he was too ill to go anywhere. Clemency panicked and broke away from the group. Lucy, Serenity, and Henry returned to the house.

Clemency tells Libby how she made it to Sally's home in Penreath. Then, Libby gets a photo on her phone, asking her to come back to the Cheyne Walk house. Clemency recognizes the woman in the picture as Lucy. Libby and Miller return to Cheyne Walk, where Libby reunites with her long-lost family members.

Henry then reveals that the person he has really been addressing his narrations to is Serenity/Libby. He claims Libby looks just like her father, and that her father is Phin, not David. Lucy confirmed that for him after Clemency ran away. He had then gone to kill Phin, who was tied to a radiator in his bedroom. Henry untied Phin and fell asleep next to him. When he woke up, Phin and Lucy were gone, leaving Henry alone with Serenity. Henry faked the suicide note left with the dead adults and left the house after giving Serenity the lucky rabbit's foot. He called 9-9-9 and pretended to be a concerned neighbor, which prompted the police to go in and find Serenity. Henry details how he had to work his way up in life and had his looks cosmetically altered to look more like Phin, as he wanted nothing to do with the weak, pathetic Henry he felt he once was. He eventually seduced an employee at the Lambs' solicitors' office and gets the man to give him the Lamb family trust. David had not managed to change it, and Henry could not prove his identity to claim the estate. He had to wait for Serenity to turn 25.

Lucy describes how she and Phin escaped and got help from the neighborhood physician, Dr. Broughton, who helped them get to France. After two years there, Phin departed, leaving Lucy on her own to busk with her fiddle for a living.

This leaves Libby wanting to find her father, and when Libby turns 26, she quits her job and goes out to celebrate with her mother and uncle. Miller reveals that he has found Phin, who works as a safari guide in Africa. Miller has tickets for himself and Libby to go visit him. Henry tells the reader that it was he who wrote "I AM PHIN" all over the house and is the one who made Phin ill with a love potion gone awry. The novel ends with Henry asking if he can join Miller and Libby on their trip to meet Phin, putting on a false air of being jolly Uncle Henry.