We can divide the acts in terms of the basic stages of Neil and Brenda's relationship. Act I lasts from Brenda and Neil's first meeting at the country club pool to the end of Neil's first week in the Patimkin house. Though there are some rough spots, their relationship is basically idyllic up to this point. The act ends on Neil's dream of sailing away from an island paradise with the little boy from the library.
The second act begins the day Harriet flies in from Milwaukee for her wedding to Ron. It consists of Neil asking Brenda to buy the diaphragm and all the resulting drama. It includes the wedding reception scene. Brenda and Neil's love affair has turned rather dark as Neil's insecurities take over and Brenda's attempts to reassure him are never enough. The act ends with an unceremonious description of Brenda's departure for school.
This act begins with Neil's return to work, and includes his loneliness and emptiness at being Brenda-less. Their relationship is already over, though neither of them has quite realized it. This act includes more diaphragm drama and the official break-up. Neil realizes that (perhaps) that the world of books might be a source of satisfaction in his life. In the end, he goes back to Newark to work at the library.