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 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.