The series follows the advances of mercenary Sam "Serious" Stone against Mental, an extraterrestrial overlord who attempts to destroy humanity at various points in time. The first game, Serious Sam: The First Encounter, was released for Microsoft Windows in March 2001. Several spin-offs were developed by other developers, such as a Palm OS conversion of The First Encounter by InterActive Vision, Serious Sam: Next Encounter (on GameCube and PlayStation 2) by Climax Solent, and Serious Sam Advance (on Game Boy Advance) by Climax London.