The Oath tracks the intertwined lives of Dr. Katz and SS Doktor Bloch, who both survive Auschwitz and the war to practice medicine in the United States, never losing the stains upon their souls. Katz, a respected Jewish French surgeon captured by the Nazis, volunteers to serve at Auschwitz, believing he can protect his wife and children from the gas chambers by helping Joseph Mengele and other SS physicians with their human studies. There he meets Tamara Lissner, a Czech teenager who miraculously survives death in the chambers, but when he hides her in his lab, she becomes witness to the medical torture Katz is forced to impose upon fellow Jews as he assists Doktor Bloch in his cold immersion experiments. After the war, Doktor Bloch avoids criminal prosecution through Operation Paperclip, a clandestine government program that welcomed German scientists to America (including Werner von Braun), ignoring Nazi ties and their participation in the war.

Years later, Tamara reenters Dr. Katz's life, helping him deal with the shame and guilt of his actions at Auschwitz. But just when Katz is coming to terms with his past, he receives a call from former Auschwitz inmate Martin Brosky, who has devoted his life to bringing painful retribution to Nazi criminals who have evaded capture. Martin insists that Michel help him kill Bloch, and Michel must now weigh the risks of defying Martin against the sweetness of revenge, knowing well that his own sordid complicity at Auschwitz could be revealed.