The play is set in the "bed-sitting-room" of a Mississippi Delta plantation, on a single evening in the summer.
As the play begins, Margaret is shouting at her husband Brick over the roar of his shower, complaining about her "no-neck monster" nephews. Brick's brother and sister-in-law, Gooper and Mae, have brought their five children to Big Daddy's home to display them like animals at a county fair, and it is driving Margaret mad. Brick is uninterested in his wife's concerns.
Margaret reveals that Big Daddy – Brick's father – is dying of cancer. The report only came that day. Margaret is certain that Gooper's family is only in town for Big Daddy's birthday because they want to get on his good side, now that he is dying. She also worries that they are trying to get Brick locked up for being an alcoholic – which he is. His leg is currently in a cast because he broke in the night before, in a drunken attempt to jump the hurtles at the high school athletic field. But Margaret thinks Brick still has the advantage because Big Daddy prefers him.
Brick lies on the bed and largely ignores Margaret as she continues talking about how Gooper thinks he married up but is still low rent. She observes that Brick is looking at her coldly, and she calls him on it, but he barely acknowledges her. She says that "living with someone you love can be lonelier than living entirely alone if the one that you love doesn't love you." Brick asks if she would like to live alone, and she adamantly insists that she wouldn't.
Margaret forces the conversation back to trivial manners, complimenting Brick on not losing his looks despite being a drinking man. But she wishes he would, because that would ease her attraction to him, which tortures her because he refuses to make love to her. She asks what is the victory of a cat on a hot tin roof, and answers the question herself – just staying on it, as long as she can.
Brick takes another drink, and drops his crutch. Margaret tries to get him to lean on her, but he refuses. Brick announces that the click hasn't happened yet, the click in his head when he's had enough to drink that he becomes peaceful. He tries to get her to shut up, and she tries to get him to sign a birthday card for Big Daddy. Brick says that she is forgetting the conditions on which he agreed to continue living with her, and Margaret replies "I'm not living with you. We occupy the same cage."
Mae enters, and asks Margaret to hide her archery set around the children, mocking Margaret for not having any of her own. Margaret in turn mocks Mae for giving her children ridiculous names like Trixie and Buster. Mae asks why Margaret is so catty, and leaves.
Alone again, Margaret cries to Brick that she is catty because she is consumed with longing for a man who won't love her. She asks when her punishment will be over, and he responds by encouraging her to take a lover. But she cannot make herself be interested in any man but her husband. He says she is making a fool of herself, but she wants to make a fool of herself over him. She cries that she cannot accept their status, and seizes his shoulders. He pushes her away and threatens her with a chair, "like a lion-tamer facing a big circus cat," but they are interrupted by Big Mama.
Brick retreats into the bathroom while Margaret greets Big Mama. It is revealed that Big Mama doesn't know that Big Daddy has cancer, nor does Big Daddy know. Big Mama asks if Brick has been drinking, and blames Margaret and her childlessness for Brick's alcoholism.
Big Mama leaves, and Brick re-enters. Margaret says that she is certain their sex life will pick up again, and shows off how well her body looks. Men constantly make advances at her, and Brick says she should respond to them, but she refuses to give Brick grounds to divorce her.
Margaret forces Brick to realize that Big Daddy is dying, and reveals that Big Mama will be told later tonight. Big Daddy will be kept in the dark. Mae and Gooper are plotting to get the better share in his will, relying on Brick's childlessness, but Margaret is intent on getting the money herself. She has been poor her whole life but she needs money for when she gets old.
Margaret adds that it was a mistake to tell Brick about Skipper. Brick says to shut up about Skipper, but Margaret refuses. She and Skipper made love so that each of them could feel a little closer to Brick. She thinks it was noble, a beautiful way to express a pair of impossible loves. Brick threatens Margaret for her implications, saying that "a man has one great good true thing in his life – I had friendship with Skipper – You are naming it dirty!"
Margaret amends that it was only Skipper who even unconsciously desired anything not "perfectly pure," and when she confronted Skipper about it, he hit her and then slept with her to prove it wasn't true. From then on, Skipper was a drunk, until drinking killed him. Wild now, Maggie cries that Skipper is dead, but she is alive, and cannot be ignored. Brick tries to hit her with his crutch, and falls.
Dixie, Gooper and Mae's daughter bursts in. Brick tells her that he is on the floor because he tried to kill her Aunt Maggie and failed. Margaret yells at Dixie, who says that Maggie's just jealous because she can't have babies, and leaves again.
Margaret tells Brick that a gynecologist said there's no reason why she can't conceive, and that it is the right time of the month for her. Brick asks how she is going to conceive with a man who can't stand her, and Maggie says this is a problem she will have to work out. As the curtain falls, the rest of the family is on their way into the room.
The first act of Cat On a Hot Tin Roof is one continuous scene, a single dialogue between Maggie and Brick, almost unbroken save for the occasional brief interruption. Many plays will compress all the action of an act into a single scene, but it is rarer for that scene to not feature the periodic re-alignment of the characters involved. Tennessee Williams makes the bold choice to show us this one conversation, trusting his stars to hold the attention of the audience all the way until the first intermission.
This scene-act is simultaneously a huge information-dump and a pivotal moment in Brick and Maggie's relationship. It is very transparently structured, building repeatedly from a stasis position of Maggie monologuing at a taciturn Brick to a heated, angry, and violent moment of truth. Each time the couple reaches this dangerous breaking point, they are interrupted by a family member. The interruption forces Brick and Maggie to retreat to their corners and catch their breath before starting the cycle over again when they are again alone. This cycle repeats at least three times, with each confrontation building on the venom of the previous, until we finally reach the breaking point that ends the act. Maggie and Brick will not be left alone again for a long time, so they are now permanently frozen at the height of their fury with one another.
Williams employs a barrage of playwrights' tricks to keep the audience's attention during this scene. Brick's silence and temper create two starkly contrasting moods – when he is silent, Maggie monologues, essentially uninterrupted. But when Brick is angered, Maggie switches gears altogether, and we get a glimpse of the couple's former chemistry while enjoying a completely different tone.
Williams also seeds the conversation with references to slow-cooking issues, heightening tension and curiosity. Skipper is mentioned long before we find out that he slept with Maggie, and we find that out long before we learn the connection that event and his death. Big Daddy's illness is revealed, and then taken back, and then re-revealed. Significantly, we learn a great deal about Big Daddy through hearsay, without ever meeting him, effectively heightening anticipation for his actual introduction in the second act.
Another difficulty in writing such a scene is in keeping it visually interesting. Two people talking in a single room for forty-five minutes can easily come off badly on stage, but Williams gives a director plenty to work with. Brick lounges on a giant bed while Maggie paces and dresses herself, physically highlighting the gulf between them. When Brick stands – when he enters her space – he is on a crutch, limping, ineffectual. It is not where he belongs, and he only passes through there to reach for more alcohol. The physicality in their arguments also works as contrast – the only time Brick reaches for Maggie is to strike her, and the visual effect of this should be jarring and cruel.
Heavy attention is given to the idea of Maggie the Cat. The play's title is explained outright in dialogue – she is a cat on a hot tin roof, just trying to stay up there as long as possible. She defends herself against accusations of being catty, and she preens and grooms throughout the act like a big Persian. Williams even carries through the imagery to the stage directions, having Brick face Maggie with a chair in arm, like a lion-tamer at the circus. This cat has got claws, and she will bare them.