Serious Sam First Encounter is a classical example of how to create a good game which one can spend hours on without ever growing tired of playing. While it is already old, it is by no means outdated. (With perhaps the exception of the graphics, but hey, that's what happens with any game if more than 10 years pass) Don't mind my lack of gametime on steam - I must have over 100 hours throughout my childhood on this game, it's amazing, and I can recognise it isn't just nostalgia making me say that.
Storywise, it's nothing to write home over. It's a fairly standard story for a game that relies more on its gameplay than on captivating you with word words and more words. Aliens happened, they're after you, you're in acient Egypt and you have guns. Good luck!
There is more to it, if you take the time to read into the story there are just enough flavourful blurbs and explanations around that you can read to figure out where your enemies came from, and parts of the storyline, though it isn't something that will carry you through the game, it's more like an extra.
The true power of Serious Sam is in the gameplay, with excellent level design, guns that are satisfying to use, and a very high skill ceiling allowing you to feel like a god laying down devastation on hordes of enemies, moving fast enough to dodge almost all of their fire (Since all but a few enemies use projectiles, this means you can dodge nearly everything with enough practice) yet not too fast to become unmanagable. This, along with plentiful secrets, and a game that won't just say 'no' and throw up an invisible wall if you move slighly off the beaten path means that Serious Sam has an excellent grasp the concept of what great games are about- Freedom
Freedom to choose your own tactics (And most are valid as well! No need to gimp yourself)
Freedom to move as you please
Freedom to explore the levels and find all the secrets
And the freedom to handle any encounter in the way you want to.
Along with this, Egypt as pictured is a somewhat unique setting for an FPS game, with a lot of it lending itself for interesting levels and impressive scenery. The enemies you face are unique and mostly iconic as well, with the beheaded bomber and kleer skeleton being some of the more known enemies. In addition, Serious Sam has a great sense of humour, and you can play with a friend, two friends, or a whole army of friends.
If you're interested in a great FPS game and don't mind slightly outdated graphics, Serious Sam is there for you.
And if you do mind the graphics, check out the HD version! It's worth it