I can only remember the rough plot of a novel that I read many (15, 20?) years ago. (I am not talking about the film "Allied".) It went something like this.
The wife in a US American married couple is actually a Russian spy. The husband works as a rocket scientist or journalist. In any case, the spy tries to steal and/or manipulate a rocket program.
In the end the man falls in love (and sleeps with) another woman who is his colleague and/or helps him in his adventure, which consists of unmasking the spy and/or clearing his name. When unmasked his wife claims that even though she's a spy, she does actually love him.