On the day they first meet in a city playground, Deborah Laidlaw lends Toby Ruben a book called Trolley Girl, the memoir of a forgotten trolley strike in the 1920s, written by the sister of a fiery Jewish revolutionary who played an important, ultimately tragic role in the events. Young mothers with babies, Toby and Deborah become instant friends. It is a relationship that will endure for decades—through the vagaries of marriage, career, and child-rearing, through heated discussions of politics, ethics, and life—until an insurmountable argument takes the two women down divergent paths. But in the aftermath of crisis and sorrow, it is a borrowed book, long set aside and forgotten, that will unite Toby and Deborah once again.