UFO: Alien Invasion is a turn-based strategy game in the style of the old X-COM PC games, where you control a secret organisation charged with protecting the world against a brutal alien enemy.
The gameplay is broken down into two parts.
"Geoscope" mode is all about high-level tasks. You'll research new technologies to help you in your battles, produce new weapons, build up bases, and launch interceptor craft to shoot down invading UFOs.
The heart of the game, though, is Tactical Mode, where you send a hand-picked team of specialist soldiers to an area where there's been reports of alien activity, and have them find out exactly what's going on.
Unusually, this works in a turn-based form. Every soldier has a certain number of "time units" to spend per turn. You might use these to move a soldier as far forward as possible across a map, for instance. But, you may find his very last step brings an alien into his sight. And so when your turn ends, the alien will be able to attack and kill him with ease.
If you're not used to turn-based games then this initially will be, well, a little frustrating. But you just have to be careful, move forward very slowly, don't just rush into rooms, allow your soldiers to cover each other, and so on. Which of course is actually what this situation would be like in real life; it all helps to create a surprising amount of tension and excitement.
UFO: Alien Invasion isn't a sequel or remake of X-COM, then, but it does generally do a good job of recreating the same atmosphere. The graphics are a little retro, but as acceptable as you'd expect from a modified version of the Quake 2 engine, and if you're looking for some engrossing and entirely free entertainment then there's a good chance that the program will appeal.