The game is set in post-doomday Earth. The majority of humanity perished to an alien biological weapon. Now the aliens are terraforming the Earth through a spreading biomass, making it more conductive for themselves while wiping out all native life on the planet. In addition, the advanced alien army itself is quickly capturing all resources on the planet.
You are the military leader of the resistance, reporting to the Council of Earth. Your job is to assemble teams of commandos to take back the planet and stop the alien biomass infestation. Each commando has different strengths and weaknesses. You have to combine different roles (soldier, medic, sniper, etc.) to achieve your objectives. The game is fairly hard, so you have to diversify and try to use your full team to take down a target quickly. Lost commandos are gone permanently, but you keep getting new recruits.
You can also research and develop a large number (nearly 70) of alien technologies, which goes on in real-time as your commando teams are battling on the field. Missions are fairly diverse, ranging from sanitizing (wiping out alien presence in) an area, search-and-rescue missions, storming and taking over crashed alien ships, acquiring alien artifacts, securing installations, capturing territory, etc.
The musical score is really excellent and sets the mood nicely. In contrast, the voice-acting is quite weird. Several of the commandos sound very odd, particularly the female commandos who sound like manly men with laryngitis. However, the graphics in the game are decent. The controls are simple to use, but you will need to pause a lot, as your commandos need to receive every explicit instruction from you.
All in all, UFO: Aftermath is an interesting game to play. If you like sci-fi games set in post-doomsday Earth you won't be disappointed.