The main quest
Pop Goes the Weasel is the main Easter egg of Mob of the Dead. It picks up where the escape plan stops: the plane always crashes on the bridge, the electric chairs always send the four inmates back to their cells, and the only way out of the loop is a fight between the mobsters themselves.
It was the first time in Zombies that players could finish a map and end the game without being killed by zombies, and the first time a playable character turned hostile to his own team.