The year is 1971, the place is California, and what never seemed possible suddenly becomes irresistible. Camping on the shores of Fallen Lake in the high Sierra, two couples swap partners and glimpse paradise. That night changes the course of their lives and their children's as the two marriages become one. The families move in together in suburban Pleasant Valley, the exhilarated adults pursuing their four-sided relationship and their dream careers. At first resentful of this upheaval, the children find unexpected advantages. But the door flung open by the sexual revolution is starting to close. Too late the lovers realize what they will have to pay for living out their fantasies.