This is the introduction/tutorial mission for the game. You play as Dalton, a 007-looking man who is trying to track down and disarm explosives in the House of Commons. You're taught basic distraction/attraction and takedown techniques, as well as setting traps and basic hacking.
You're also introduced to Sabine, who seems to be the current leader of DedSec, as well as Bagley, who is a computer/AI who will ultimately disarm the explosives. Once this happens, you will learn there are three other locations rigged to explode, and you are then tasked with getting to the roof of the building to shut down the transmitter. Once this happens (or so you think), you'll be introduced to Zero-Day, and a cutscene plays that detonates those three explosions throughout London and begins the story with the introduction video and title sequence, during which the achievement should pop.
All in all, it should take 15 minutes or so to get through.