The Love Interest has noted something odd about her partner. He is gone at all hours, does strange things and never answers a straight question. Is he betraying her?
Never fear! Your love interest still has Undying Loyalty toward you and would never do such a thing. No, it is something else. Rather he is a great patriot bravely defending his country against The Empire. She, of course, can't be told about this because she has no need to know. So she must wonder.
Not necessarily a married couple, of course. Subtrope of Mistaken for Cheating.
- The plot of True Lies is an inverted example. Spy and counter-terrorist agent Harry Tasker thinks his wife Helen is cheating on him, though she doesn't seem to suspect him of anything other than being a boring salesman. She is sort-of seeing a guy, who claims to be a spy but is, in fact, a car salesman.
- In Head Over Heels, Amanda and her friends are suspicious of Amanda's new boyfriend Jim. When they investigate, the evidence leads them to believe that he is a violent criminal. He's actually an undercover officer.
- Justified in The Assignment (1997). An American naval officer who happens to be the doppelganger of Carlos the Jackal is used for an intelligence operation where he must pose as Carlos well enough to fool a girlfriend of the notorious terrorist. This includes instruction from an ex-lover of Carlos in how he has sex. The officer's wife accuses him of cheating because he's suddenly changed his personal habits and sexual technique.
- Old Russian joke inversion: "If your wife has cheated on you, be thankful she betrayed you, not the Motherland."
- In the NCIS episode "Power Down," a young man discovers that his recently murdered wife, who he momentarily thought was cheating, was a security agent working to protect The Government against cyberattacks.
- A subplot arc in the first season of Spooks involved the girlfriend of one of the agents adding two and two together to get five. (The time the agent went undercover as a married man and then forgot to take the ring off before he went home really didn't help.)
- Spy's Suspicious Daughter in The Americans. Paige, the teenage daughter of two Soviet deep cover agents masquerading as Americans, grows suspicious of her parents in Season 2 and starts snooping.
- Mary Woodhull, Abraham's wife in Turn.
- Not a spy, exactly, but Bones once had a victim who turned out to be separated from her husband because she (and her sister, who also suspected him of the murder) thought he was cheating. It turned out he was sneaking out and spending time at a sleazy motel to meet a legal contact because he was testifying in a major federal case.
- One episode of CSI involved the investigation in the murder of an undercover cop that had been lynched by the criminal biker gang that he had infiltrated and the apparent fact that this had happened because his cover had been blown. Turns out that his wife had been suspicious for a long time now of him being unfaithful to her and called the number on his cell phone that she believed was his mistress' while pissed-off and blindingly drunk. Which was the number of the gang's roadhouse. While the cop was in the room. And the call was put on speaker. Oops.
- This is a Real Life danger of the espionage profession.