A game from 1998, and it still looks good ? Seems impossible, but it is possible.
You play as black mesa facility scientist named Gordon Freeman, you arrive late to your work. Everyone in black mesa seem to be talking about the experiment what is testing an mysterious crysital from unknown place. As you expected, everything goes to hell, experiment failed, aliens from XEN world are released to world and HECU marines are sent in to sabotage the experiment and a mysterious man with an suit and an briefcase keeps on watching you.
If you haven't played half life before, then there are many reasons why you should. Great creative weapons, good gameplay, advanced AI, a lot of great mods what extend the replay value and you can get to understand the later series(half lfie 2) story a lot better.