Wilson learns that his wife has a lover. In the chapter, Tom stops at Wilson's garage for gas, driving Gatsby's car. Tom is there with Jordan and Nick. Wilson is sick with the knowledge that Myrtle has a life outside of the one that they share; he is determined to move away with her. This is why he is so insistent on getting Tom's old car—he thinks he can sell it for a profit.
There is a diabolical symmetry in Tom's and Wilson's situations, despite all of Tom's upper-class snobbery. Like Wilson, Tom has only just realized that his wife has been unfaithful (Daisy is in love with Gatsby), and his anger over this causes him to brag to Wilson about the yellow car he is driving—despite the fact that the car is, in fact, Gatsby's.
Like Wilson (who locked Myrtle in the upstairs of the garage until he can finalize his plans to move), Tom has "locked up" Daisy, forcing her and their whole party into a kind of "imprisonment" at the Plaza hotel. Gatsby's flight with Daisy from New York happens at the same time Myrtle escapes from her imprisonment and rushes into the street. When Daisy then hits and kills her with Gatsby's car, it not only marks the end of Tom's affair with Myrtle, but also marks the end of Daisy's affair with Gatsby.
By the end of the chapter, Tom has reasserted his control over Daisy, and Gatsby—still in love with her—is then left waiting anxiously outside the house: "watching over nothing."