Serious Sam is a first-person shooter done by Croteam in 2001. Unlike other similar games, it doesn’t rely on naturalistic game worlds and high realism, but you’re supposed to stop the intergalactic arch villain Mental and his henchmen from conquering the universe. A remake of this game called Serious Sam HD was released in 2009.
In development since 1996, Serious Sam was originally supposed to demonstrate Croteam’s game engine, the “Serious Engine”. However, it has been praised so much that the publisher Gathering of Developers sponsored a complete game based on it. In this video, you can see an early beta version that has been published about a year before the final release. It was meant to test hardware compatibility and gameplay problems, not to show up.