This is the third book in a trilogy. A portal opens up to an alternate earth where the human species is Neanderthal rather than Homo sapiens. In the first two books, a relationship developes between Ponter Boddit, the Neanderthal who came through the portal, and Mary Vaughn, the geneticist who was called to study him. Neanderthals have "man-mates" with whom they live 24 days a month and "woman-mates" with whom they live when "Two become One" four days a month.
In this book, Mary and Ponter determine whether they can have a child, and how they can live in both worlds under the Neanderthal retrictions, now that a permanent portal has been established. Mary, a devout Catholic and Ponter, with no concept of religion, must also resolve spiritual issues.
On this earth, the magnetic field is undergoing a collapse and reversal and the consequences are unknown. Jock Kreiger, head of the Spectrum think tank studying the Neanderthal world, realizes the "other" earth is unspoiled and beautiful and wants to take over. Also working in the think tank is Cornelius Ruskin, a geneticist who worked with Mary at York University and raped her and another faculty member when they received promotions he felt he was entitled to.
In the previous books, Mary learned who raped her and Ponter punished Cornelius in his world's manner- castration. Jock uses Cornelius to try to take over the Neanderthal world using genetic means. Ponter, Mary and their friends work to counter his plan, and the earth's magnetic field begins to change.
This report prepared by Joan Zuckerman