This is the first true story mission in the game if you exclude the tutorial.
It will be very clearly marked on your map as what looks like a white factory.
If you do the tutorial it will be in the first region that you make contact with. You scan that region and then the mission will be available. Doing so will also give you:
The mission itself is fairly simple. I will give a quick spoiler-free rundown; there is no timer on the mission so you can take your time.
- You start outside the facility, high-up away from patrols.
- Make your way until you come to a road and a parked advent ship, there will be several pods to fight here, you'll have an easier time if you try and take them one at a time.
- Once past the road there should be another pod near the entrance of the facility. Lots of cover here, grenades can be helpful.
- After the fight here, prep your squad and stack up against the doors.
- Once inside fight at least one squad and take your time to recover.
- Once you're ready, take most of your squad outside ready in some strong positions and have one member recover the item
- Be prepared for taking out a pod or alternatively rushing to the evac.
- When the mission is complete the achievement will pop and you will also cause the aliens to lose some avatar progress.
Hope this helps.
P.S. Some comments - if you're playing a high difficulty or even any difficulty it can be worth holding out on completing this for sometime as it is considered one of the easier missions in the game as the enemies don't scale up and is therefore ideal for an emergency reduction in avatar progress. This is especially relevant with the new expansion as some of the other facilities can be guarded by an alien ruler!