The play takes place on a country estate owned by Sorin, a retired senior civil servant in failing health. He is the brother of the famous actress Arkadina, who has just arrived at the estate for a brief vacation with her lover, the writer Trigorin. Sorin and his guests gather at an outdoor stage to see an unconventional play that Arkadina's son, Konstantin Treplyov, has written and directed. The play-within-a-play features Nina, a young woman who lives on a neighboring estate, as the "soul of the world" in a time far in the future. The play is Konstantin's latest attempt at creating a new theatrical form, and is a dense symbolist work. Arkadina laughs at the play, finding it ridiculous and incomprehensible; the performance ends prematurely after audience interruption and Konstantin storms off in humiliation. Arkadina does not seem concerned about her son, who has not found his way in the world. Although others ridicule Treplyov's drama, the physician Dorn praises him.
Act I also sets up the play's various romantic triangles. The schoolteacher Medvedenko loves Masha, the daughter of the estate's steward. Masha, in turn, is in love with Konstantin, who is in love with Nina; Nina does not return his feelings. When Masha tells the kindly old doctor Dorn about her longing, he helplessly blames the lake for making everybody feel romantic.
Act II takes place in the afternoon outside of the estate, a few days later. After reminiscing about happier times, Arkadina becomes engaged in a heated argument with the house steward Shamrayev and decides to leave immediately. Nina lingers behind after the group leaves, and Konstantin shows up to give her a seagull that he has shot. Nina is confused and horrified at the gift. Konstantin sees Trigorin approaching, and leaves in a jealous fit. Nina asks Trigorin to tell her about the writer's life; he replies that it is not an easy one. Nina says that she knows the life of an actress is not easy either, but she wants more than anything to be one. Trigorin sees the seagull that Konstantin has shot and muses on how he could use it as a subject for a short story: "A young girl lives all her life on the shore of a lake. She loves the lake, like a seagull, and she's happy and free, like a seagull. But a man arrives by chance, and when he sees her, he destroys her, out of sheer boredom. Like this seagull." Arkadina calls for Trigorin, and he leaves as she tells him that she has changed her mind – they will not be leaving immediately. Nina lingers behind, enthralled with Trigorin's celebrity and modesty, and gushes, "My dream!"
Act III takes place inside the estate, on the day when Arkadina and Trigorin have decided to depart. Between acts Konstantin attempted suicide by shooting himself in the head, but the bullet only grazed his skull. He spends the majority of Act III with his scalp heavily bandaged. Nina finds Trigorin eating breakfast and presents him with a medallion that proclaims her devotion to him using a line from one of Trigorin's own books: "If you ever need my life, come and take it." She retreats after begging for one last chance to see Trigorin before he leaves. Arkadina appears, followed by Sorin, whose health has continued to deteriorate. Trigorin leaves to continue packing. There is a brief argument between Arkadina and Sorin, after which Sorin collapses in grief. He is helped off by Medvedenko. Konstantin enters and asks his mother to change his bandage. As she is doing this, Konstantin disparages Trigorin and there is another argument. When Trigorin reenters, Konstantin leaves in tears. Trigorin asks Arkadina if they can stay at the estate. She flatters and cajoles him until he agrees to return to Moscow. After she has left, Nina comes to say her final goodbye to Trigorin and to inform him that she is running away to become an actress, against her parents' wishes. They kiss passionately and make plans to meet again in Moscow.
Act IV takes place during the winter two years later, in the drawing room that has been converted to Konstantin's study. Masha has finally accepted Medvedenko's marriage proposal, and they have a child together, though Masha still nurses an unrequited love for Konstantin. Various characters discuss what has happened in the two years that have passed: Nina and Trigorin lived together in Moscow for a time until he abandoned her and went back to Arkadina. Nina never achieved any real success as an actress, and is currently on a tour of the provinces with a small theatre group. Konstantin has had some short stories published, but is increasingly depressed. Sorin's health is still failing, and the people at the estate have telegraphed for Arkadina to come for his final days. Most of the play's characters go to the drawing room to play a game of bingo. Konstantin does not join them, and spends this time working on a manuscript at his desk. After the group leaves to eat dinner, Konstantin hears someone at the back door. He is surprised to find Nina, whom he invites inside. Nina tells Konstantin about her life over the last two years. She starts to compare herself to the seagull that Konstantin killed in Act II, then rejects that and says "I am an actress." She tells him that she was forced to tour with a second-rate theatre company after the death of the child she had with Trigorin, but she seems to have a newfound confidence. Konstantin pleads with her to stay, but she is in such disarray that his pleading means nothing. She embraces Konstantin, and leaves. Despondent, Konstantin spends two minutes silently tearing up his manuscripts before leaving the study. The group reenters and returns to the bingo game. There is a sudden gunshot from off-stage, and Dorn goes to investigate. He returns and takes Trigorin aside. Dorn tells Trigorin to somehow get Arkadina away, for Konstantin has just killed himself.