Mycroft is a control freak, he likes to control things, including people's perception of him. He has been obsessively maintaining his image, the cool, calm, impenetrable governemnt official that everyone fears.

That's excactly how Lestrqde saw him as, though instead of fearing him, Lestrade has been pining after him like a teenage boy. He always wanted to break the calm and see the man trembling, and begging beneath his body.

Then Mycroft goes into heat, out of the blue, while, perhaps, having 'small chat' with Lestrade. Mycroft freaks out and tries to conceal the fact he's in heat. But it's futile, of course.

Thankfully, Lestrade is a beta, so he's not 'chemically' affected by Mycroft's heat, but the sight of the man coming undone, desperately and shamelessly begging him to fuck him is enough to drive him crazy too.

Even though Mycroft is in heat, he's fully aware of his surroundings, and what he's exactly doing right now and he's extremely ashamed of himself for giving up his control so easily now.

Lestrade fucks Mycroft, of course, but not without teasing and humiliating him a little, because since he's a beta, he isn't yet completely taken over by his urges, so he fucks him inch by agonizing inch, ignoring whimpering, begging Mycroft, until Mycroft finally admits, yes, he's indeed a cock slut, and that Lestrade's cock tastes fucking amazing.