There's a Thin Line Between Love and Hate (Felinette spy au)

When rival spies Felix Graham de Vanily and Marinette Dupain-Cheng-code names Cat Noir and Ladybug-are forced to go undercover as a married couple to infiltrate an infamous criminal's exclusive gala, neither of them expects the mission to be anything more than a battle of wits. Felix is mysterious, cocky, and stubborn; Marinette is sly, seductive, and lethal with a smile. Both determined to be Vanguard's top agent, their cutthroat rivalry soon twists into undeniable attraction as they dodge deadly threats, maneuver high-stakes deals, and pretend to be passionately in love. But as lines blur and the danger escalates, Felix and Marinette realize the only way out is to trust each other-for both the mission's success and the desires they can no longer ignore.