Act I
As Picnic opens, Millie is sneaking a cigarette outside, while Hal and Mrs. Potts have just finished breakfast next door. It is the last day before school starts and everyone is getting ready for a Labor Day picnic to be held that evening. The main characters are introduced, and the tomboyish Millie, while pretending not to care, is shown to be envious of her older sister Madge's beauty but contemptuous of her intellect.

Madge enters. Hal, a drifter recently arrived in town and employed doing odd jobs for Mrs. Potts, is immediately attracted to her, and Madge is clearly attracted to him. When Flo enters, her wariness indicates that she perceives Hal as a threat to her plans, mainly marrying Madge off to the wealthy Alan. Flo warns Madge that a pretty girl does not have much time before her beauty begins to fade, urging her daughter to seize the moment and secure Alan's interest. Madge enjoys Alan's company but is not in love with him. Instead, she is fixated on the train whistle in the distance, which, to her, symbolizes the prospect of freedom from the small town.

The Owens' roomer, Rosemary, enters the scene and attempts to convince the other women that she is not interested in men or in marrying. But her tone indicates that marriage is the one thing she does desire. She leaves with two other single teachers. Alan arrives and embraces Hal as an old friend from college. The two reminisce, and Hal relates his most recent activities. Alan's acceptance of Hal eases Flo's worries about the drifter and validates Mrs. Potts's fondness for her new handyman. Hal is asked to escort Millie to the picnic that evening, and the act ends with Hal, Alan, and Millie leaving to spend the day swimming.

Act II
It is late afternoon, and Madge has spent the day helping her mother prepare the food for the picnic that evening. Rosemary is getting ready for her date with Howard, who soon arrives. Hal and Alan have also arrived in two separate cars. After Alan goes inside the house to help Flo, Millie sits down to draw Hal's picture. Howard leaves to get something from his car, returning with a bottle of whiskey. After initially pretending that she is unfamiliar with alcohol, Rosemary has several drinks, as does Hal and Howard.

When Rosemary walks to the other side of the yard, both Hal and Howard step back to admire Madge, who they can see primping in the window. Rosemary returns and wants to dance to the music everyone can now hear coming from the park. Although Howard has told Rosemary that he cannot dance, he makes an attempt to please her after she begins dancing with Millie. Hal attempts to teach Millie some new steps; Rosemary is fixated on him as he swoons to the music. When Madge enters, and she and Hal begin to dance, Rosemary, already very drunk, begins a verbal attack on Hal, who has refused to dance with her. Rosemary correctly assesses Hal's social skills and his insecurities. Her attack leaves him defeated, and when Madge tries to comfort him, he embraces her, carrying her offstage.

Act III, scene 1
It is very late, after midnight, when Rosemary and Howard return. They have had sex, and Rosemary expects Howard to marry her. Howard's attempt to escape, by repeatedly promising that they will talk about their future at another time, is not acceptable to Rosemary. She makes it clear that even one more day of the loneliness and emptiness of her life is unsuitable. Although he has claimed to be set in his ways and unwilling to marry, Howard is no match for the determined Rosemary. She pleads and begs until he promises to return in the morning to discuss the issue. A few moments later, Madge and Hal enter. They have also had sex, and both are feeling very ashamed at their betrayal of Alan. The scene ends with Madge running into her house in tears.

Act III, scene 2
It is early the next morning. Everyone is carefully discussing the events of the evening before. Alan arrives and asks to speak to Madge. While he waits, Millie tells him that she has always liked him. As they are waiting for Madge to come outside, Howard arrives to speak to Rosemary, who has packed her bags and is prepared for an immediate wedding. Using the presence of her friends and the Owens, whom she has certainly told of her wedding plans, Rosemary effectively traps Howard into agreeing to an immediate wedding.

Madge comes outside to speak to Alan, who tells her that Hal has left town. But Hal has been hiding nearby, and his entrance provokes a fight between the two young men. Hal easily defeats Alan, who says he is leaving town with his father. Hal is being pursued by the police and must also leave town. He tries to convince Madge to go with him, but she is frightened. He leaves to catch the train to Tulsa without her. After Hal leaves, Madge enters the house crying. She emerges in a few moments with her packed bag and announces that she, too, is going to catch the train and join Hal. In spite of her mother's pleadings, Madge walks offstage.