Brick enters, and Mae and Gooper mock him. Gooper then presents Big Mama with some legal papers he wants her to sign. The papers would leave Gooper and Mae in charge of Big Daddy's estate after (or perhaps even before) his death. Big Mama refuses even to look at them. She says that no one will get anything until Big Daddy himself lets go of it.
As a storm brews outside, Big Mama embraces a reluctant Brick, who has remained aloof from the family argument. Big Mama seems to acknowledge for the first time that Big Daddy is dying. She tells Brick that the best thing he could do for Big Daddy would be to produce a grandson for him.
Big Daddy's voice is heard on the gallery, and then he enters the room. He wants to know what the big discussion was about, and asks about the contents of the envelope that Gooper is putting back into his briefcase, but Gooper will not say. Big Daddy then tells a lewd joke about two elephants in a zoo, and comments that he smells mendacity in the room. He gets Brick to agree with him. Then he addresses some cruel remarks to Big Mama and Maggie.
Margaret kneels at Big Daddy's feet and announces that she is pregnant. That is her birthday present to Big Daddy, she says. Big Mama rejoices, while Big Daddy tells Gooper that he wants to see his lawyer in the morning. Then he exits, saying he is going up to the roof to survey the plantation before he has to give it up. Big Mama follows him.
Mae says she knows Maggie is lying, but Maggie counters that she has seen one of the best gynecologists in the South. Neither Mae nor Gooper believe her. Mae says that she knows that Brick does not sleep with Maggie.
A cry of agony and rage fills the house, and is then repeated. It comes from Big Daddy.
Mae and Gooper exit angrily, leaving Brick and Maggie alone. Brick is still drinking, and he finally gets that "click" he wants, that makes him feel peaceful. He goes out onto the gallery where he sings a peaceful song. Maggie empties the liquor cabinet and follows him out. Then they both return. She turns out all the lights except the one by the bed. She says she really has seen a doctor and she knows it is the right time of the month for her to conceive. She tells him she has removed all his liquor, and that that night they are going to make the lie she has told come true. When they've done that, she says, she will bring the liquor back and they will get drunk together. Brick has nothing to say as she turns out the light. Maggie says she loves him, and he replies with the enigmatic remark that echoes his father's words in the previous Act, "Wouldn't it be funny if that was true?"
Most scenes in the play contain examples of dramatic irony, and this last scene is no exception. With the exception of Brick, who knows otherwise, the family believes that Big Daddy still does not know that he is dying of cancer. But of course he does, because Brick told him. So when Big Daddy stomps around the room talking about "mendacity," no one but Brick knows what he is talking about.
In spite of everything that has happened, mendacity has not been eliminated from the family. Maggie shows she is prepared to lie to get what she wants, although she has every intention of making the lie come true. Brick seems to be willing to go along with her plan, without embracing it with any enthusiasm. In spite of the cathartic encounter with his father in the previous Act, he does not seem to have changed much. In this Act, he remains on the sidelines, gradually getting drunk and refusing to offer his mother the support she needs. When it comes to making love to Maggie, it seems that he has finally given up trying to resist her. She has proved to be the stronger of the two, and it is her will that prevails. She is the tenacious cat that has managed to stay on the hot tin roof.