Shell (Michelle Pine), a 33-year-old graphic designer, has recently lost her job at a design agency and broken up with her long-term partner, Gav. Back in her parents' home in the Donaghmede housing estates of Northside Dublin, she is directionless and broke. While grocery shopping at the Woodbine Crown Mall, a dilapidated suburban shopping center built in the late 1970s, she spots a handwritten HELP NEEDED sign in the window of a small florist. Inside, she meets Neve, the shop's owner: a tall woman with cropped hair, scarred palms, small glasses, and a dark ring on her left hand. Shell is immediately attracted to her. Neve offers a full-time assistant position starting at four a.m., warns the Crown will likely close within the year, and Shell accepts on the spot.
What Shell does not know is that the mall harbors something far more dangerous than structural decay. An unnamed, sentient plant entity narrates portions of the novel in first person, revealing itself to be a predatory organism whose vines thread through the walls and floors of the entire building, radiating outward from the Green Hall, a large glass terrarium at the center of the atrium. Neve has called the plant Baby since she was a teenager. Baby's ultimate goal is to consume Neve's heart, symbolized by the dark ring she wears, and to merge permanently with her. It views Shell as the instrument through which this can be accomplished.
Shell proves a fast study at the shop, and her Instagram account, repurposed to showcase her bouquets, attracts thousands of followers. She meets Neve's circle of friends: Daniel Kavanagh, a flamboyant hairdresser who recognizes Shell from school; Bec, a polished travel agent; and Kiero, a quiet 22-year-old who runs the mall's copy shop and whose older sister, Caoimhe, died under mysterious circumstances. Neve invites Shell to biweekly after-hours gatherings in the food court, and Shell feels a sense of belonging she has not experienced in years.
Through Baby's narration, the novel reveals Neve's history with the plant. Baby first emerged from the earth in the Green Hall when Neve was 16. Her aunt Carla, who ran the flower shop, tended it alongside Neve until Baby grew large enough to feed and consumed Carla entirely, absorbing her body, memories, and senses. Rather than destroy the plant, Neve carried Carla's remains from the building and let others find the body. She left school to take over the shop, and no suspicion ever fell on her. Over the following years, Baby fed again: It dragged a drunk man into the Green Hall, consumed a five-year-old child who wandered behind a shelf in the supermarket, and killed Kiero's sister by poisoning her through a corsage. Neve was furious and forbade Baby from harming anyone in her circle again.
Baby also recounts how Neve's ex-partner Jen, an American ethnobotanist who moved to Ireland for Neve, left months earlier because Neve refused to relocate when the Crown's closure was announced. After Jen's departure, Neve wore their engagement ring, woven from strands of both their hair, on a chain near her heart. Baby has coveted this ring for years, believing it contains Neve's heart, but Neve has always refused to surrender it. Each night, Neve unlocks the Green Hall, tends to the plant, and lets its vines embrace her, a ritual that has sustained their bond for over 15 years.
Shell's attraction to Neve deepens alongside a flirtation with Kiero, who gives her a book with his phone number inside. Shell tells Neve she does not plan to date men again, a deliberate signal Neve registers but does not act on. When Kiero leads Shell through the mall's hidden service corridors, Baby drops a vine from the ceiling. Shell touches it, and Baby fuses microscopic threads of itself into her skin. The next morning, tiny orchid buds bloom from her palm and cheek. She rushes to Neve, who extracts the growths with tweezers and antiseptic, standing between Shell's knees to work on her face. Neve explains that a plant in the Green Hall, big and clever, has chosen to reveal himself to Shell. She warns Shell to tell no one, and their dynamic shifts to one of co-conspiracy.
In a parallel thread told through emails, Jen writes to Bec from the Burren, revealing her fear that something dangerous is growing in the Green Hall. She recalls the morning Neve knelt before the orchid and spoke to it as though it were a person and remembers hidden cleaning supplies and the smell of something like blood. She proposes driving to Dublin to investigate.
The Crown's closure accelerates when vendors receive an email announcing permanent shutdown in one week. Shell proposes converting her Instagram following into an online delivery business; Neve agrees to relocate. Beneath Neve's grief, Baby detects something unexpected: relief and a desire to be free. Enraged, Baby resolves to act quickly.
Shell attends a dinner with her old college friends, who stage a rehearsed intervention accusing her of selfishness. With a push from Baby's influence, Shell smashes her wine glass and walks out, blocking her old social circle from her phone.
Neve brings Shell into the Green Hall to formally meet Baby. The terrarium, transformed by Baby's power, glows with golden light, wisteria, and birdsong. Baby opens its eye and speaks; Shell whispers that she believes. Neve kisses Shell for the first time, and during the kiss, guided by Baby's silent instruction, Shell slips the ring from Neve's chain without Neve noticing. That evening, Shell sleeps with Kiero at the beach, and when she climaxes, white flowers bloom through her hair. Fearing that Baby or the place will destroy him, she tells Kiero he needs to leave Ireland.
Separately, Jen drives to Dublin and enters the shuttered mall with Bec. They photograph vines growing through ceiling tiles, discover a room of shed white petals, and reach the Green Hall, where Jen takes a cutting from Baby with lab scissors. On the drive home, the cutting grows explosively: Baby fills the car with moss and vines, kills Jen, and drags the car into the earth. Jen somehow revives underground, claws her way to the surface, and stumbles to a pub. Bec drives across the country in the middle of the night to rescue her.
The next morning, Baby instructs Shell to go to the Green Hall and bury the stolen ring in the earth beneath its altar. Relieved to surrender decision-making to Baby's authority, Shell obeys. Baby summons Neve, who arrives and sees Shell performing the act Baby has begged Neve for years to do herself. The ring makes contact with Baby's core, and the plant splits Neve open, bursting forth in a violent eruption of petals. Baby and Neve merge into a single entity, their voices becoming a shared "we." Neve's body is subsumed into flower, then rot, then nothing. Shell witnesses the transformation and goes catatonic.
Bec and Jen return to the Crown the next morning and find Shell vine-covered but alive. Jen goes to the Green Hall with a shovel and a jug of caustic chemical Neve had kept for controlling Baby. She digs the merged organism from the ground, ignoring Neve's voice crying from within, and places it into a bucket of the solution. The Woodbine Crown begins to collapse around them, and the three women sprint to the car park. Bec and Jen instruct Shell to stay and explain Neve's death to authorities, then Bec drives south toward the Burren with Jen and the bucket.
The following spring, Shell works at a flower shop in Galway City, sharing a flat with her old school friend Cara. Kiero has emigrated to London, Daniel freelances across Dublin, and Bec works at Cassidy Travel's head office. They stay connected through a group chat called The Woodbine Crown Survivors' Club. In her lab in the Burren, Jen keeps Baby, now a twin-blossomed orchid with a mismatched pair of eyes, sealed in a glass tank. She mails Shell a pressed leaf from the plant. Shell places pieces of it under her tongue before bed, and behind her closed eyes, the world turns green: Neve appears, smiling, holding Baby, his roots spilling down her body. They look at Shell as though she were perfect.