The Mother-in-Law by Sally Hepworth chronicles the turbulent family dynamics of the wealthy Goodwin family. Published in 2019, the crime thriller explores the complexities of motherhood as Lucy Goodwin struggles to uncover the truth behind the tragic death of her mother-in-law, Diana Goodwin. Hepworth’s sixth novel, the book features multiple narrators and flashbacks that offer a suspenseful examination of Lucy’s self-discovery while grappling with the violent death of her family’s complicated matriarch.

Content Warning: The Mother-in-Law depicts murder and discusses death by suicide.

Plot Summary

While preparing dinner at home, Lucy and Ollie Goodwin are notified by two police officers of the sudden death of Ollie’s mother, Diana Goodwin; the police suspect she died by suicide. Lucy and Ollie uncover more details about Diana’s death, and Ollie, skeptical, informs the police of Diana’s breast cancer diagnosis.

Ten years earlier, Lucy meets the Goodwin family for the first time in their opulent home. Overwhelmed by their wealth, Lucy struggles to connect with the cold and distant Diana. Still, Lucy hopes to find a maternal figure in her, since Lucy’s own mother died when she was 13. When Lucy and Ollie become engaged, Lucy invites Diana to shop for wedding dresses. A philanthropist and advocate for refugee women, Diana is disgusted by the cost of Lucy’s dress and scolds the sales associate for referring to her as Lucy’s mother.

In the present, Lucy and Ollie meet with Ollie’s sister Nettie and her husband Patrick, wondering who would have a motive to kill Diana. In a flashback to Lucy’s wedding day, Diana lends her a Celtic knot necklace as her “something borrowed” and reminds her to return the necklace after the wedding. A year later, Diana refuses to help a pregnant Lucy and Ollie afford a home. Ollie’s childhood friend Eamon tries to convince Ollie to partner with him in business, but Ollie refuses.

In the past, Diana drives to the hospital to meet her first grandchild, Archie. On the way, she receives a call from a young refugee woman named Ghezala, who is also in labor. Diana abandons her plans, delivers Ghezala’s baby at her home, and gives her a teddy bear intended for Archie. Two weeks later, Diana arrives at Lucy’s home with a raw chicken and leaves before explaining its significance.  

Following Diana’s death, Ollie does not return to work at the recruitment agency he and Eamon started two years ago. Lucy discovers a bank statement detailing the company’s tremendous debts. Before Lucy confronts Ollie, two detectives arrive and reveal that Diana did not have breast cancer and that they are investigating her death as a murder.

In flashback, Lucy discovers that Ollie convinced his father Tom to go against Diana’s wishes and give him money for a down payment on a home, Lucy demands that Ollie never seek financial support from his parents again. Diana explains her resistance to lending Ollie the money. Diana became pregnant with Ollie at 20. Raised Catholic, she planned to put her baby up for adoption but later decides that she wants to raise her baby as a single mother. After her own mother ostracizes her, Diana runs away to the home of her divorced cousin Meredith.

In the present, Lucy notices bruises on Nettie’s wrist. In the past, Diana recalls a visit from Nettie after Archie’s birth, when Nettie was upset about her infertility.

During a family trip to the Goodwin family’s beach house, Lucy brings a baby monitor to dinner, but Diana turns it off. Furious, Lucy and Ollie leave with Archie the next morning. A year later, the family reconvenes Diana’s home. Lucy is pregnant again, and Ollie announces his new business venture with Eamon. Lucy and Diana both question Ollie’s decision. Diana visits Ghezala, also pregnant with her second child, and argues with Ghezala’s husband, Hakem, who accuses Diana of not understanding the struggles of refugees as a privileged woman.

In the present, Lucy and Ollie report to the police station to review Diana’s autopsy results. The detectives question them separately. Lucy is asked about a violent incident between years ago. Diana was found with a bottle of Latuben, a drug used in euthanasia, but no trace of it was found in Diana’s body. The detectives believe Diana’s murderer smothered her with a pillow.

In flashback, Lucy gives birth to her second child, Harriet. Lucy struggles with life postpartum, and when Diana offers judgmental advice, Lucy grows frustrated.

Diana’s lawyer informs the family that Diana has left her entire estate to her charity. Lucy remembers once dropping her kids at Diana’s house and overhearing Hakem thank Diana for helping him find a job. The two women bond over their belief in raising children to be contributing members of society.

In a flashback, Nettie confides in Lucy about her infertility and her hopes for IVF, which Diana refuses to pay for. At a tumultuous Christmas, Nettie complains about her mother’s lack of financial support. Diana defends herself and criticizes Lucy’s choice to be a stay-at-home mother. After Christmas, Diana feels isolated from her children, who refuse to speak with her. Two days before Christmas, Diana discovered Patrick’s infidelity, and this is why she doesn’t want to pay for IVF. When Nettie reconciles with her, Diana decides not to reveal Patrick’s affairs. Ghezala visits Diana, who shares what happened after she gave birth to Ollie. Taken in by her estranged cousin, Diana adapted to new motherhood and began helping Meredith in return. Once, exhausted, Diana fell asleep, forgetting to buy a chicken. Meredith bought the chicken for Diana to allow her to rest.

In the present, Eamon argues with Ollie until Ollie begins to choke Eamon. After Eamon leaves, Ollie confesses to Lucy that they will likely go bankrupt.

Three months after Christmas, a pregnant Lucy reaches out to Diana and asks her to watch Archie and Harriet. Diana ignores Lucy’s warning not to go swimming. At the pool, Tom accidentally drops Harriet, fracturing her skull, Infuriated, Lucy pushes Diana to the ground at the hospital.

At Diana’s funeral, Lucy wears the Celtic knot necklace Diana willed to her and cries openly. She meets Ghezala and Hakem, now serve on the board of Diana’s charity. Talking with Ghezala makes Lucy feel more connected to Diana.

The morning after the incident in the hospital, Diana observes Lucy caring for Harriet and feels like she understands Lucy more. When Tom is diagnosed with ALS, Diana flashes back to when she found work in a movie theatre that allowed to bring Ollie. There, she reunites with Tom, then a young plumber whom she met at her parents’ home. Lucy arrives at Tom and Diana’s home to apologize. An emotional Diana informs Lucy of Tom’s diagnosis.

In the present, a neighbor claims to have seen Ollie at Diana’s home on the day of her murder. Lucy calls Nettie to help with her children as the police question Ollie. Nettie seems preoccupied with her fertility issues and not with her mother's death.

In a flashback. Lucy finds Diana hiding in a closet at Tom’s funeral. She comforts Diana and takes control of the funeral. Grieving, Diana experiences depression and Lucy drives her to a doctor who prescribes her antidepressants. When the doctor refers to Lucy as Diana’s daughter, Diana does not correct the doctor.

In the present, Ollie confesses to Lucy that he did visit Diana on the day of her death. Lucy is asked to go to the police station for questioning about Diana’s work with an organization called Voluntary Euthanasia International.

In a flashback, Nettie and Patrick update Lucy on their fertility journey. Nettie asks Lucy to serve as an egg donor and surrogate. Lucy refuses.

Still experiencing symptoms of depression, Diana discovers the work of Voluntary Euthanasia International. She learns how to acquire the drug Latuben. In the present, the police question Lucy again. A week after attending a Voluntary Euthanasia meeting, Diana lies about a breast cancer diagnosis and asks Lucy to help her encrypt email addresses and purchase bitcoins.

Diana meets with Nettie, who begs Diana to fund her surrogacy plan. Disturbed by Nettie’s mental state, Diana agrees to consider it. Later, Patrick asks Diana to refuse to fund the surrogacy. Diana confronts Patrick about his affairs and demands that he tell Nettie. He insults Diana’s parenting and threatens her. After Lucy and Diana express how much they care for one another, Diana abandons her plans for voluntary euthanasia.

On the day of her murder, Diana meets with Ollie, who asks for Diana’s help with his failing business. Diana offers to be a silent partner if Ollie begins a new business venture, and he agrees. Diana’s idea is for a recruitment agency for refugees. Later, Nettie arrives to discuss her surrogacy plan, and Diana reveals Patrick’s affairs. Undeterred, Nettie pressures Diana to give her the money. Diana confesses her suicide plan, trying to empathize with Nettie’s pain. When Diana refuses to fund the surrogacy, Nettie suffocates her with a pillow.

In the moments following Diana’s death, Nettie retrieves the Latuben and pours it down Diana’s throat. Later, she uses the leftover Latuben to die by suicide. Local newspapers write salacious articles about Diana’s murder and Nettie’s death.

Ten years later, Lucy and Ollie work at their refugee recruitment agency, funded by Diana’s charity. Despite continued financial struggles, they are happier and more fulfilled than ever. As Lucy and Ollie interview their last candidate for the day, she glances at a framed picture of Diana’s suicide letter and reflects on how much she and Ollie have learned from Diana.