A young man, Dan, takes a young woman to hospital after she has been hit by a taxi; they flirt as they wait for the doctor to attend to her bloodied knee. Larry, a dermatologist, inspects her leg briefly and leaves. Dan and the young woman introduce themselves—he is Daniel Woolf, an obituary writer and failed novelist who tells her how he and his colleagues use euphemisms humorously in their work in obituaries. At the young woman's prompting, he says his euphemism would be "reserved" and hers would be "disarming". She is Alice Ayres, a self-described waif who has a scar along her leg shaped like a question mark. Wanting him to spend the rest of the day with her, she calls in sick to his office for him.
More than a year later, Dan is on the verge of publishing a book based on Alice's past as a stripper, and Anna is taking his photograph for publicity. Dan is infatuated with Anna, though he is in a relationship with Alice, for whom he left his former girlfriend. He begs Anna to see him again, and she rejects him. Alice overhears his conversation with Anna. She asks Anna to take her photo, and when Dan has left, confronts her. Anna insists she is "not a thief" and snaps a photo of a tear-stricken Alice.
Six months later, Dan and Larry meet in an adult chat room. Dan impersonates Anna and has Internet sex with Larry. He tries to play a practical joke on Larry by arranging for Larry to meet him (Dan pretending to be Anna in the chat room) in the London Aquarium the next day. When Larry arrives, stunned to see Anna (who Dan didn't know would be there), he acts believing that she is the same person from the previous night's internet chat and makes a fool of himself. Anna catches on and says that Dan was probably playing a practical joke on him. She reveals that it is her birthday and snaps a photo of Larry.
At Anna's exhibition of photos, Alice stands in front of the photo of her, looking at it; Dan is watching her. They have an argument over Alice's feeling that Dan will leave her. Larry meets Alice, whom he recognises as the woman in the photo, and knows that she is Dan's girlfriend. Meanwhile, Dan convinces Anna to carry on an affair with him. They cheat on their partners with each other, even through Anna and Larry's marriage. Finally, a year later, they tell their partners the truth and leave to be with each other.
Alice, devastated, disappears from Dan's life. She returns to stripping, using the name Jane. Larry finds her at one of the seedy strip clubs in London, where he pushes her to tell the truth about her name. In a poignant moment, he asks, "Tell me something true, Alice." She says, "Lying is the most fun a girl can have without taking her clothes off— but it's better if you do." They share a connection based on mutual betrayal and heartbreak. He asks her to meet him later for sex. She declines.
A month after this, Anna is late meeting Dan for dinner. She is coming from asking Larry to sign the divorce papers. Dan finds out that Larry had demanded Anna have sex with him before he would sign the papers. Dan becomes upset and jealous, asking Anna why she didn’t lie to him. They have a candid, brutally truthful conversation. Anna reveals that she did have sex with Larry and he did sign the papers.
Alice meanwhile has been having sex with Larry. On his birthday, she summons him to the museum and sets up Anna to meet him there. Larry and Anna exchange words, as Anna discovers Alice and Larry have been having a casual relationship. Larry asks Anna if their divorce will ever become finalised; he leaves when Alice emerges. The two women share a heated exchange in which their mutual animosity is revealed. Anna calls Alice "primitive", a description Alice accepts. The younger Alice describes Larry's emotional state as troubled and learns from Anna that Dan still calls out for "Buster" (Alice's nickname) in his sleep.
Anna goes back to Larry. Distraught, Dan confronts Larry at his office and has to reconcile himself to the loss of Anna. Larry recommends Dan go back to Alice and reveals that he had seen her in the strip club. He lies for Alice at first and tells Dan that they did not sleep together, since Alice feared that, if Dan found out, he would not want her any more. At the end, Larry decides to hurt Dan and reveals the truth—that they had slept together.
Dan and Alice, back together, are preparing to go to America. They relive the memories of their first meeting, but Dan is haunted by their encounters with Larry and Anna and pushes Alice to tell him the truth. In the moment when Alice becomes caught between refusing to tell the truth and being unable to lie to him, she says, "I don't love you anymore. Goodbye." (She had told Dan in the beginning that these are the words she tells her significant others when their relationship is over and she is going to leave.) She tells Dan to leave. He struggles with her; she spits in his face, and he throws her back on the bed, grabbing her neck. She dares him to hit her, and he does; she leaves.
Later, Anna and Larry meet again, only to reveal that they have broken up once again and Larry is dating a young nurse named Polly. They are meeting because Alice has died the night before in New York, having been hit by a car while crossing the street. Their conversation also reveals that "Alice" was always a pseudonym—her real name was actually Jane. Larry leaves as Dan arrives because he has patients to see. Dan talks with Anna and says that no one could identify Alice's body and he is flying over to America to do so. Before Dan leaves, he tells Anna that Ruth, his ex-girlfriend whom he left for Alice/Jane, is now married, has a child, and is pregnant with a second. She married a poet, having fallen in love with him (without ever having met him) after reading his book of poems, Solitude. Dan and Anna bid each other a cold goodbye. Dan leaves to catch his flight, leaving Anna alone.
