On a summer evening in London, Christine and Alex Klimec, a married couple in their fifties, are listening to music when the telephone rings. Christine answers to hear her oldest friend, Lydia, calling from the hospital. Lydia's husband, Zachary Samuels, has collapsed at his art gallery office and is dead from an apparent heart attack. Pausing outside her studio door, Christine finds the idea of her art suddenly fraudulent. She locks the studio from the outside and pockets the key, marking the beginning of a months-long creative paralysis.
Alex is devastated. After the music finishes, Christine leaves for the hospital, where she finds Lydia sitting alone. Back at the flat, Alex insists on driving through the night to Glasgow to tell Grace, Zachary and Lydia's daughter and an art student, in person. On their return, Grace goes straight to Isobel, Christine and Alex's daughter and a civil servant, and the two friends embrace while their mothers stand frozen. Grace announces she wants to see her father's body and make a death mask, horrifying Lydia. The household settles into an uneasy vigil. Sandy, Alex's son from his first marriage to Juliet and now a famous musician, arrives. In the middle of the night, Lydia climbs into Christine and Alex's bed, shivering. Christine puts socks on her feet and holds her; Alex embraces her too.
An extended flashback covers the four characters' twenties. Lydia and Christine had recently graduated from university, where Alex, married to Juliet, had taught them French. Alex's family had emigrated from Czechoslovakia in 1968 when he was nine; his novelist father, Tomas, never wrote again after leaving Bratislava. Lydia became obsessed with Alex and inserted herself into his household as a babysitter, bringing Christine along. Christine was pursuing a PhD on Christina Rossetti while secretly making scraperboard drawings, images scratched from an inked board, depicting everyday objects that suggest human presence through absence. Alex discovered the drawings and urged her to show them to his closest friend, Zachary, who encouraged Christine to abandon her PhD and take her art seriously. Christine and Zachary began a relationship, but she recognized that Zachary was in love with Lydia. She told Lydia, who eventually married Zachary. Alex declared his feelings for Christine, and despite her fear of his intensity, they became a couple.
In the present, the community honors Zachary at Garret's Lane, the 18th-century Huguenot chapel he converted into a gallery. Grace photographs her father's body using wet-plate collodion, an early photographic technique, but the images are so haunting she asks Alex to hide them. The funeral fills the gallery with tributes, including Sandy playing a guitar piece he composed for Zachary. Afterward, Lydia moves into Christine and Alex's spare room. Christine stops working entirely, carrying the studio key hidden in her handbag. The household sews a memorial quilt from Zachary's cut-up suits with a scarlet Z at its center. Alex grows uncomfortable with Lydia's presence; Christine defends her friend. One afternoon, Christine catches Lydia reading Alex's book of poems with intense concentration.
A second flashback covers the mid-nineties, when Zachary used his inheritance to establish the gallery. Christine became pregnant but lost the baby at 10 weeks; though she grieved, she was half-relieved, recognizing the pregnancy would have consumed time from her art. Hannah, the gallery's manager, privately told Christine her paintings were not prominent enough for the opening show, a wound Christine never revealed. Alex secretly trained as a schoolteacher. Christine accepted money from Zachary to fund extra painting hours, keeping the arrangement secret from Alex. Christine resented that Alex never truly took her art seriously. At the gallery's opening night, the four friends began kissing and touching one another before the fire, until little Isobel appeared at the staircase. The encounter was never repeated; Christine and Alex discussed it privately but never raised it with Zachary and Lydia.
Returning to the present, the night Alex drives back from Glasgow, Christine sends Lydia home and prepares the flat for his return. At two in the morning, she calls Lydia, who hesitantly reveals that Alex is there. He had let himself into Garret's Lane expecting empty rooms but found Lydia playing music. They embraced first for comfort, then with desire, and made love. Alex drives to Christine's flat, but she has bolted the door. She packs Lydia's belongings into bin bags and remembers that she recently moved the studio key to a blue coffee pot on the kitchen shelf, but cannot bring herself to enter the studio, afraid her work will be a failure.
The aftermath unfolds painfully. Christine has the locks changed and packs Alex's suitcases. Isobel refuses to speak to her father. Alex rents a bedsit in Gospel Oak. Lydia tracks Christine to a Barbican concert and sits in Alex's empty seat. During the interval, she tells Christine that Alex mostly does not come to her and that she spends time waiting and afraid. She reminds Christine that she loved Alex first. Christine stiffly says she is looking forward to the second half.
A third flashback takes the four friends to Venice. While the others were in Vicenza, Zachary and Christine visited the Tiepolo ceiling in the
Scuola dei Carmini, a Venetian confraternity hall. Zachary confessed he felt like everyone's brother, even Lydia's, hinting at dissatisfaction in his marriage, and proposed they make love. They did. That evening, Zachary used Christine's work to argue that women artists may be exempt from certain gendered self-doubt. Alex's dismissive reaction revealed he did not truly take Christine's art seriously, a discovery that dismayed Zachary.
In the present, Christine repaints the flat and builds a life without Alex. He visits, and they talk carefully, but she tells him the flat is no longer his home. Alex drives away impulsively, spending nearly a week unreachable. In his absence, Lydia reads Alex's anthropology books and discovers genuine intellectual engagement for the first time. Alex returns to Lydia unannounced. She proposes they leave Garret's Lane, buy a new place, and make room for Grace. She wants to find work and be more involved with the gallery. Alex agrees.
Isobel reveals to Christine that she is pregnant by her boyfriend, Blaise, a civil servant from an upper-class background. She is determined to keep the baby. Christine panics briefly but calms herself, remembering that time for art can always be found. She sends Lydia a dawn email describing a dream and a memory of Lydia's wit in their school days, telling her not to reply and not to be sorry.
Christine visits the last exhibition Zachary arranged at the gallery but finds herself bored by the abstract paintings. When Hannah asks what she is working on, Christine lies, claiming she will show something soon. At home, she stretches out on the sofa, playing with an idea for a new series: small studies of a woman on a bed beside an open window. She retrieves the studio key from the blue coffee pot and opens the door. The room smells of turps and linseed oil; a window has been left open for months and the lightbulb has blown. She steps inside in near-darkness, feeling her way among her materials, and experiences a frightening promise of relief and happiness. Brushing her fingertips across the thick paper on her desk, she thinks that she has at least made the first mark, and has begun something.