Half-Life 1 in a nutshell (you can stop it at the Half-Life 2 part):
http://www.youtube.com/watch?v=devEDGtgZ64
Basically, you are scientist/badass Gordon Freeman. You work for super secret Black Mesa. Things go haywire, aliens come. Stuff blows up. You blow stuff up. You fight crazy alien boss. You get hired by the G-man. Credits roll. You're not missing much (other than the best first person shooter EVER made).