The central mystery of the film—why Brick hates Maggie and why Skipper killed himself—is slowly unraveled in the third act. It is revealed that Skipper, feeling inadequate when Brick was away, tried to prove himself by making a pass at Maggie. Maggie coldly rejected him and forced Skipper to confront the truth: that he was desperately, unrequitedly in love with Brick. Distraught, Skipper called Brick, who hung up on him in a panic, leading to Skipper's suicide. Brick's anger at Maggie is actually a projection of his own intense guilt for abandoning his friend.
In the film's climax, the ultimate "mendacity" is shattered when Big Daddy learns he does not have a spastic colon, but terminal cancer. Unlike the play, the film features a deeply sentimental reconciliation in the basement where Big Daddy and Brick bond over their mutual failures. Upstairs, to stop Gooper from taking the estate, Maggie announces a bold lie: she is pregnant with Brick's child. While in the play Brick remains indifferent to this lie, the film provides a Hollywood twist: Brick admires Maggie's "desperate truth," fiercely defends her against his brother's skepticism, and leads her upstairs to their bed to consummate their marriage, ensuring the lie will soon become a reality.