It’s difficult to identify a linear progression in the campaign of XCOM due to its expansive RTS gameplay mechanism, but there are a certain set of missions that are mandatory or essential to progress through the story-mode.
These essential missions have specific prerequisites that must be carried out before they can be played. This guide will walk you through the requirements of the missions, along with general and specific tasks that you would need to do during the missions.
Hidden Base Assault
How to Unlock:
In order to gain access to this mission, you will need to construct the Alien Containment Facility. Once you have constructed this facility, you will need to capture a Sectoid and an Outsider, and attempt to research them.
To capture the aliens, you will need to gain access to the Arc Thrower, which can be used to stun Aliens and capture them for interrogation and autopsy.
The Hidden Base:
The Base itself will be your first serious challenge in the game. The structure of the hidden base boasts a lot of tight corridors with blind corners, which can lead to aggressive Chrysslids. Be sure to check corners at the start of your turns, or you will surely lose a few men if you are reckless.
Drones will also appear alongside Chrysslids in some places. Make sure that you take them out fast to nullify their healing abilities.
Apart from Drones and Chrysslids, you’ll encounter Floaters as well.
But the real challenge of the mission doesn’t come until the very end. You will face your first Sectoid Commander here. He has a Mind Control ability which can disintegrate even the most organized squad. The Mind Control has a very long range, and it will turn your soldiers into foes. You obviously don’t want to kill your own people, so it is extremely important to actually put all your focus on the Commander.
You will have to take the risk of getting out of cover and focusing all of your fire on the Commander. Make sure he is dead before he gets too dominating with some mind controlled friendly soldier.
How to Unlock
You must unlock and complete the Hidden Base Assault before you can play this mission. Using the technology you recover from there, you should have the capability to build a Hyperwave Relay. This key facility will allow you to see the Overseer ship.
The Overseer Mission:
In addition to the unlocks, I would highly recommend that you do research for your Interceptors, as this ship is fast, powerful, and hence very difficult to bring down. You should also get some boost items like UFO Targeting and some defense related stuff.
Once it is down, you will have to send in a squad to capture the device within. The ship will introduce you to a set of intimidating enemies. You will find some beefy Sectopods, Heavy Floaters, and a super-dangerous mind-controlling Ethereal that is protected by a set of Muton Elite.
You should set up an ambush, and lure these enemies one by one to that ambush – if you take them all at once it will get severely overwhelming.
Take this into consideration particularly when fighting the Ethereal and his Muton Elite friends. You DO NOT want to be taking on them simultaneously unless the situation absolutely demands it. The Ethereal in particular is very strong, being much more powerful than the Sectoid Commander and also having a powerful Mindfray blast.
The Temple Ship
How to Unlock:
The Overseer Mission is a prerequisite for the Temple Ship. However, this final essential mission requires a lot more than just the completion of the Overseer mission.
You have to research the device captured during the mission. In addition to that, you must build PSI Labs, and find a gifted psychic soldier through Psionic Testing in the Officer Training School. You will also develop the Gollop Chamber, and then have the psychic soldier communicate with the Gollop Chamber to gain access to the final mission.
The Temple Ship Mission:
You’ll face all kinds of enemies in the Temple Ship. In fact, the starting area (where you spawn) is very tricky and difficult itself, because right when you pass the narrow door you will be attacked by some Sectoids, Cyberdiscs, and Drones.
Don’t blindly run through the door here, as you will certainly be caught in fire despite the seemingly good cover there. Afterwards, you will face some Chrysslids, in addition to Thin Men, and then immediately followed by a squad of Mutons.
After facing the Mutants, the large room beyond will consist of Muton Elites and Sectopods. You will need some HEAT or Shredder missiles for those Sectopods, and the Muton Elites can be dealt with with some smoke.
The final battle is against the Uber Ethereal, which is accompanied by a few other Ethereals. There are some Muton Elites there as well. This is where you should use your mind control to control the Muton Elites. This will allow you to lob an alien grenade to the chamber where the Uber Ethereal is.
Make sure you keep your distance from the Uber Ethereal, and take headshots with the possessed Mutons as much as possible.