Mother Night is the story of Howard W. Campbell, Jr., an American recruited to be an Allied spy in World War II-era Germany. Campbell works as a Nazi radio propaganda artist, sending encoded messages to the Americans over the air during pro-Hitler programs; the problem is that only one man knows Campbell is really working for the Americans, and after the war, the rest of the world views him as a traitor and an anti-Semite. This complex novel deals with themes of identity and wartime morality.
As the Nazi Party consolidates its power over 1930s Germany, Campbell decides to remain in the country despite his parents' decision to leave. Campbell continues to write plays, his only social contacts being members of the Nazi Party. Being of sufficiently Aryan parentage, Campbell becomes a member of the party in name only. He is politically apathetic, caring only for his art and his wife Helga, who is also the starring actress in all of his plays.
The first part of the book ends after Campbell visits the Berlin Zoo and encounters Frank Wirtanen, an agent of the U.S. War Department. Wirtanen wants Campbell to spy for the United States in the impending world war. Campbell immediately rejects the offer, but Wirtanen quickly adds that he wants Campbell to think about it. He tells him that Campbell's answer will come in the form of how he acts and what positions he assumes once the war begins.
Once World War II starts, Campbell begins to make his way up through Joseph Goebbels' propaganda organization, eventually becoming the "voice" of broadcasts aimed at converting Americans to the Nazi cause. Unbeknownst to the Nazis, all of the idiosyncrasies of Campbell's speeches– deliberate pauses, coughing, etc. – are part of the coded information he is passing to the American Office of Strategic Services. Campbell never discovers, nor is he ever told, the information that he is sending.
About halfway through the war, Helga goes to the Eastern Front to entertain German troops. Campbell is extremely distraught when he hears that the camp Helga visited in Crimea has been overrun by Red troops and she was presumed dead. In early 1945, just before the Red Army invades Berlin near the war's end, Campbell visits his in-laws one last time. During the visit, Campbell has an exchange with Helga's younger sister, Resi, that resonates with him for years afterward. After Campbell is captured by American forces, Wirtanen works out a deal in which Campbell is set free and given passage to New York City.
In New York City, circa 1960, Campbell lives a lonely, anonymous life, sustained only by memories of his wife and an indifferent curiosity about his eventual fate. His only friend is George Kraft, a likewise lonely neighbor—who, through an extraordinary coincidence, also happens to be a Soviet intelligence agent. He tries to trick Campbell into fleeing to Moscow by publicizing the fact that Campbell has been living in the United States since the end of the war.
A white supremacist organization discovers his existence and makes him a cause celebre , inviting him to speak to new recruits as a "true American patriot". The group's leader, a dentist named Lionel Jones, shows up at Campbell's apartment with a surprise: a woman claiming to be Helga, alive and well and professing her undying love. Campbell's will to live returns for the first time in years and remains even after he finds out that she is not Helga, but rather her younger sister Resi. They plan to escape to Mexico City after attending one of Jones' fascist meetings.
There, Wirtanen makes an appearance to warn Campbell of Kraft's plot and Resi's complicity. Heartbroken, Campbell decides to go along with the charade. He confronts Kraft and Resi, the latter swearing her feelings for him are genuine. The FBI then raids the meeting and takes Campbell into custody, while Resi commits suicide by taking a cyanide capsule. As before, Wirtanen uses his influence to have Campbell set free. Once Campbell returns to his apartment, however, he realizes that he has no real reason to continue living, and decides to turn himself in to the Israelis to stand trial.
The book ends as it began, with Campbell sitting in an Israeli jail cell awaiting his trial. Coincidentally, he meets Adolf Eichmann and gives him advice on how to write an autobiography. He then is transferred to a different holding cell where he further awaits his trial. At the very end of the book, Campbell inserts a letter that he has just received from Wirtanen. The corroborating evidence that he was indeed an American spy during World War II has finally arrived, and Wirtanen writes that he will testify to Campbell's true loyalties in court. Rather than being relieved, Campbell feels "nauseated" by the idea that he will be saved from death and granted freedom when he is no longer able to enjoy anything that life has to offer. In the last lines, Campbell tells us that he will hang himself not for crimes against humanity, but rather for "crimes against himself".