"The First Encounter was the first game in the series which had been in the making since 1996 and had a video showing its early game play in 2000. The First Encounter was originally developed by Croteam as a demonstrator for their engine and this is the reason why some countries saw its initial release priced at less than half the value of other games in the genre.
The game introduction reveals what happened before the game actually takes place: in ancient times, Earth was involved in a conflict between Mental and the Sirians, an alien race that left many of its artifacts to be found by humanity. In the 22nd century Mental's forces return to Earth hell-bent on eradicating humankind: as a last resort the usage of the "Time Lock" is decided: this Sirian device can send back through time a single individual who can, hopefully, defeat Mental and alter the course of history. Because of his bravery in fighting the monsters, Sam "Serious" Stone is chosen to use the "Time Lock". As the game opens, Sam appears in ancient Egypt: his travels from one Egyptian monument to the next, all the way fighting against the monsters Mental sent after him, make up most of the story of The First Encounter. During his wandering, Sam is able to activate a hidden Sirian communicator, located in Luxor, that summons to Earth a starship from the depths of space. Sam goes to the Great Pyramid for a rendezvous with the ship and, after a titanic battle against Mental's general Ugh Zan III, he is whisked on-board the Sirian starship."
Source: Wikipedia, "Serious_Sam," available under the CC-BY-SA License.