It could not have gone worse. After the devastating attack of the Reticulans in UFO: Aftermath, you only had fifty years of rest until they once again tried to eradicate the human race in UFO: Aftershock - and that time they succeeded. Only around ten thousand people survived this desperate war and only a few were allowed to remain on a station orbiting Earth. Most of the survivors were banished to Mars by the Reticulans.
At first it was only a small base and many needed to stay in a cryogenic sleep since the base could not support many lives. So time passed. The population on Mars grew and the contact with the people on Earth remained. Then scientists on Mars discovered traces of an ancient alien civilization and suddenly the whole galaxy turned its head towards Mars, but there is also something lurking underneath the surface. As leader of the colonists, it is your goal to make sure your people survive and find out what powerful technologies wait to be uncovered.
The gameplay does not differ much from its predecessors. You still manage and expand your base, train your soldiers and fight semi-real-time battles (which you can pause at any time) against various enemies. Since this is Mars, life is now much harder. You now need to look out for your water supplies and acquire new sources of water on Mars to survive. There are also several factions you have to deal with, either by force or by diplomacy, and they are very interested in your findings. If you play your cards right, you can use this to your advantage to get friendly aliens on your squad. The human soldiers gain experience, level up and can be trained to get better in certain areas.
Once you have acquired a target or get attacked, it gets down to turn-based micro management which is the heart of the game. During these parts you control your squad mates with waypoints and other orders. On the surface, the actions seems to be real-time, but in reality all gameplay still comes down to raw numbers rendering those fights highly tactical.
Many factors come into play from the plain issue of "is the target in range?" to complex problems like "my trooper is low on health and the aliens are in weapons' range. Do I risk being shot while running to cover or do I take the time to apply a medkit and thus risking a higher chance of catching a bullet?". Since the action can be paused at any given time, you have all day to make those essential decisions. They become more difficult as your squad members get more experienced, as you cannot afford to lose your veterans.