Mycroft and Lestrade have been in an on and off relationship for more than a decade. Mycroft is in love with Lestrade and wants him to break off the marriage, but Lestrade doesn't. He waits patiently, while pretending not to be in love with him.
Sherlock finds out about it, and sets out to get his revenge on Lestrade for using his brother as an outlet for his sexual frustration(whether or not Lestrade has been actually using Mycroft is up to you.)
TW: infidelity
Mycroft and Lestrade have been in an on and off relationship for more than a decade. Mycroft is in love with Lestrade and wants him to break off the marriage, but Lestrade doesn't. He waits patiently, while pretending not to be in love with him.
Sherlock finds out about it, and sets out to get his revenge on Lestrade for using his brother as an outlet for his sexual frustration(whether or not Lestrade has been actually using Mycroft is up to you.)