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 doctor in dermatology, 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. Upon the girl'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 which is shaped like a question mark. Wanting him to spend the rest of the day with her, she calls his editor and tells his boss that he's sick and can't come in to work.
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 falls in love with Anna, though he is in a relationship with Alice, having left his former girlfriend for her. 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 actually be there), he acts under the impression that she is the same person from last night and makes a fool of himself. Anna catches on and explains that it was probably Dan playing a practical joke on him. She reveals that it is her birthday and snaps a photo of Larry. They become a couple.
At Anna's showing, Alice stands in front of her photo, looking at it; Dan is watching her. They have an argument over Alice's presentiment 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, one year later, they tell their partners the truth and leave their respective partners for each other.
Alice, devastated, disappears from Dan's life and goes back to stripping, going by 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 tells him, "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, but we later learn she does actually go home with him after all.
A month after this, Anna is late meeting Dan for dinner. She's come 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, and it is revealed that Anna did in fact have sex with Larry and he did sign the papers.
Alice meanwhile has been sleeping 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 paints a pathetic picture of Larry's emotional state and gleans 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 come to terms with the fact that Anna no longer wants him. 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 telling the truth (which she refuses to do) and being unable to lie to him, she falls out of love with Dan and 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. Dan 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 hits her; 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. 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) by reading his book of poems called Solitude. Dan and Anna tell each other "goodbye" somewhat coldly and Dan leaves to catch his flight, leaving Anna alone.