AU where Stiles is an undercover FBI agent who’s supposed to be taking down some Irish mob and Derek is an undercover cop who’s doing the same thing.
Except neither of them know that the other is also law enforcement. They fall for each other so hard and Stiles keeps thing about how bad it’s going to suck when they finally take them down because then he won’t have Derek anymore. And Derek is heart broken that someone as amazing as Paddy (Stiles undercover name because) is on the wrong side of this.
Stiles almost warns Derek about the FBI/Boston PD raid.
Derek nearly pulls Stiles out when he runs out the back door in an ‘attempt to escape’ the raid.
It isn’t until Derek sees one of the FBI guys uncuffing Stiles at the station and handing him a badge and gun that Derek realized that Stiles was also undercover.
Stiles knew before Derek because he asked his partner if Derek McHale had a record and Erica had just laughed and told him Derek was Boston PD.
They end up making out in the locker room until Captain Boyd comes in and tells them to break it up. They do, but only until they get to Derek’s apartment.