This story takes place after Age of Ultron.
The team is still together and as strong as ever. One day, Nick Fury and Maria Hill walk into the tower. They explain that the team is going on a mission in Russia. That's nothing out of the ordinary, they've been all over the world. That is, until Fury explains the mission.
Vladimir Barkov. Leader of a Russian gang called 'The Russian Rebels.' The man is guilty of illegal human experimentation. He's got enhanced set up around the country. Fury doesn't know how many. He's also got assassins set up everywhere.
To even get close to killing this man, they've got to kill his henchmen. The entire time, they can't let the Russian Rebels know they're in Russia or they'll fail the mission and in best case scenario, they'll be killed.
"Sitting in this hotel room, staking this guy and his workers out.. I'm losing my mind! This mission-- it's chipping at my walls. I don't think I can rebuild them."