Mycroft and John seem to have a tentative bond of some sort over attempting to keep Sherlock from self-destructing, and after Reichenbach they stay in touch out of habit more than anything. To distract John from his grief and depression, Mycroft offers John a job as a MI6/spy/whatever and John takes to it like a duck to water.
Sherlock comes back from "death" and is stunned to find John and Mycroft are not only still in contact and somewhat friendly, but also systematically hunting down and destroying Britain's enemies with terrifying efficiently.
Jealous doesn't even begin to cover it.
TL;DR: After Sherlock "dies" John and Mycroft go on a BAMF world tour of sorts. Sherlock is not pleased. Pretty much looking for John+Mycroft almost-friendship and maybe some Sherlock/John icing.
The BAMF Club
Sherlock comes back from "death" and is stunned to find John and Mycroft are not only still in contact and somewhat friendly, but also systematically hunting down and destroying Britain's enemies with terrifying efficiently.
Jealous doesn't even begin to cover it.
TL;DR: After Sherlock "dies" John and Mycroft go on a BAMF world tour of sorts. Sherlock is not pleased. Pretty much looking for John+Mycroft almost-friendship and maybe some Sherlock/John icing.