But in order to gain access to Sirius, Sam is asked to storm Kronor, a moon orbiting Sirius that has a massive cannon which could be used to remove the protective shield on Sirius, which was a last line of defense for the Sirians against Mental. When the medallion is finally complete, Sam will then be ready to begin the final assault against the greatest enemy of humanity: Mental, who is located on Sirius, once the planet of the great Sirians that visited Earth many times, now the domain of Mental himself.
Confused at the moment and with nothing better to do, Sam accepts the mission. The Council instructs Sam to visit all five planets in order to recreate the medallion, only then will Sam become "The One". All the planets (except the planet Kleer) are populated by friendly, bobble-headed humanoids, but the problem is that all the planets are under Mental's control. The Council reveals to Sam that he must collect all five pieces of an ancient Medallion, each held by various groups on five different planets, and states that once Sam has the entire medallion, Mental will be vulnerable.
The game begins with Sam being summoned before the Sirian Great Council, where the Council provides him with guidance on how to accomplish his goal to defeat Mental.
The game's story picks up shortly after the end of Serious Sam: The Second Encounter, with Serious Sam continuing his goal to defeat his arch nemesis, Mental.