Lucy, a thirty-eight-year-old woman mired in depression after a breakup, arrives in Venice Beach, California, to spend the summer dog-sitting Dominic, the diabetic foxhound belonging to her half-sister Annika. Annika, nine years older and Lucy's only maternal figure since their mother's death, made millions selling bamboo yoga mats and married Steve, an investor. The couple is traveling through Europe. Lucy has fled Phoenix, where she works as a university librarian while languishing in a nine-year PhD program at Southwest State University on the ancient Greek poet Sappho. Her thesis argues that the vast gaps in Sappho's surviving text should be read as intentional negative space, but Lucy privately knows the premise is flawed. Her advisory committee has given her until fall to produce a draft or lose her funding.
Lucy's crisis began months earlier when she impulsively suggested breaking up with Jamie, her boyfriend of eight years, a geologist who made documentaries about national parks. To her shock, Jamie agreed. As Lucy spiraled, she discovered he was dating Megan, a younger scientist. Lucy showed up at Jamie's house and broke his nose. A police officer delivered a warning: The couple would not press charges if Lucy agreed to therapy. Shortly after, Lucy took nine Ambien, not consciously intending suicide but wanting to vanish, and was later found by police parked in the middle of the road after driving in a blackout. Annika intervened, arranging the summer stay and enrolling Lucy in group therapy for women with depression and love addiction.
In Venice, Lucy begins attending sessions led by Dr. Jude, a warm but platitude-prone therapist who instructs her to observe ninety days without dating, sex, or contact with Jamie. The group includes Amber, a combative young woman Lucy privately nicknames Chickenhorse, who frames every conflict as victimization; Sara, a woman in her fifties detoxing from a long entanglement with a noncommittal man named Stan; Brianne, a woman in her fifties pursuing men fruitlessly on dating sites; and Claire, a British redhead who left her husband for younger men and struggles with self-harm. Lucy is skeptical of the group's language of inner children and radical acceptance but reluctantly complies.
One midnight, sitting on the ocean rocks near Annika's house, Lucy encounters Theo, a strikingly beautiful young man with brown hair and iridescent teeth, swimming alone in the dark. He is warm, curious, and surprisingly knowledgeable about Sappho. Over several nighttime meetings, they develop an intimate rapport, discussing death, loneliness, and poetry. Meanwhile, defying Dr. Jude's instructions, Lucy creates a Tinder profile and goes on disastrous dates: one with Adam, an aspiring writer whose aggressive kissing and grimy apartment repel her, and another with Garrett, a handsome graphic designer who has sex with her on a hotel lobby bathroom floor and leaves without saying goodbye. These experiences leave her feeling degraded yet desperate for more male attention.
Back at the rocks, Lucy's connection with Theo deepens. He tells her she is a soft darkness, someone unusually aware of death. He massages her ankle and foot with startling tenderness. Lucy tries to stay away, recognizing her own addictive patterns mirrored in Diana, a new group member who compulsively sleeps with young tennis instructors, and in Claire, whose romantic entanglements continue to spiral. For a few days Lucy achieves balance, walking Dominic, attending group, and working on her thesis. Then, without deliberation, she returns to the rocks.
Theo performs oral sex on Lucy, and she experiences a transcendent stillness, her mind going quiet for perhaps the first time in her life. When she asks him to come out of the water, he warns her about his body. He pulls himself onto the rock and reveals that below a cloth sash, his skin gives way to scales that thicken into a massive black tail ending in translucent fins. He is a merman. He explains that his kind live for centuries, preserved by saltwater, and that the myths about merpeople killing humans are slander. Lucy is stunned but not repelled.
Their relationship becomes intensely physical. Lucy proposes bringing Theo to Annika's house in a wagon so they can be together indoors. To make this possible, she begins sedating Dominic with dog tranquilizers from Annika's cabinet, escalating the doses over time. Inside the house, Theo acknowledges carrying a deep sadness and mental exhaustion despite his centuries of life. They establish a pattern of every-other-day visits. On visit days, Lucy works productively on her thesis, which has fundamentally shifted: She now writes about desire in Sappho's surviving text, arguing that the feeling has endured like eternal love despite time's destruction. On in-between days, she hides under the covers, comparing herself to someone with an addiction waiting for the next fix.
When Lucy confesses she will be leaving Venice in three weeks to return to Phoenix, Theo reacts with devastation, saying he had planned to invite her to live with him underwater. Despite her attempts to backtrack, he drags himself across the floor and out the door, crawling across the beach to the ocean without looking back.
Lucy is ravaged by withdrawal, returning to the rocks nightly, vomiting and shaking. She visits Claire, now hospitalized after a suicide attempt, who advises her to sit with herself rather than replace Theo. Lucy discovers that Jamie has gotten Megan pregnant and they are moving in together, eliminating her last hope of returning to him. She returns to group, where Dr. Jude shares privately that the real question is not what love is, but whether love is really what Lucy is looking for. The advisory committee also informs Lucy that her new thesis direction, while superior, can no longer be funded because it incorporates personal narrative, effectively ending her nine-year academic career.
After four nights, Theo resurfaces. He reveals he watched Lucy every night from the deep ocean, waiting until she appeared fully surrendered. He asks her to come live with him underwater, presenting a rope she must use to tie herself down so her body will not fight to the surface. Lucy asks for a few days to settle her affairs. That night, she finds Dominic dead in the pantry, his body surrounded by vomit. Her escalating sedation has killed him. Annika and Steve fly home immediately. The guilt is crushing: Lucy recognizes she was given pure love in this dog and destroyed him to pursue her fantasy.
On the eve of her planned descent, Lucy and Theo share a final intimate encounter on the rocks. But as he explains the logistics, Lucy asks about the others. After pressing, Theo admits that seventeen women's bodies lie beneath the waves. Lucy's fantasy of singular, transcendent love collapses. She sees that his insatiable need mirrors her own: No number of lovers will ever fill his emptiness, just as no amount of male attention could ever fill hers.
Lucy tells him she never wants to see him again and watches him dive into the ocean for the last time. She walks back across the beach, returns to Annika's house, and tells Steve she is staying. Climbing onto the sofa, she reflects that the nothingness has returned, but it feels different now. It is hers to hold. She notices she has missed her period for over five weeks but does not pursue the thought. She turns off the light.