The game's story picks up shortly after the end of Serious Sam: The Second Encounter, with the hero of the series, Sam "Serious" Stone, continuing his goal to defeat his arch-nemesis, Mental. The game begins with Serious 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 Council reveals to Sam that he must collect all five pieces of an ancient Sirian medallion, each held by various groups on five different planets, and states that once Sam has the entire medallion, Mental will be vulnerable. All the planets (except Kleer) are populated by friendly, bobble-headed humanoids, but the problem is that all the planets are under Mental's control. The Council then instructs Sam to visit all five planets in order to recreate the medallion, only then will Sam become "The One". Confused at the moment and with nothing better to do, Sam accepts the mission.
M'Digbo (populated by Simbas) - A tropical paradise that is spoiled by Mental's forces. Here, Sam recovers the first part of the medallion after he defeats his first titanic enemy, Kwongo(A gigantic gorilla).
Magnor (populated by Zixies) - A world of giant trees and objects that make Sam feel like a toy soldier. Here, Sam conquers the second part of the medallion after he defeats Zumzum(A gigantic bee).
ChiFang (populated by Chi Che) - A planet that looks like a Chinese land and features many oriental stereotypes. The third part of medallion is received here after Prince Chan is killed by Sam.
Kleer (populated by Kleers) - The homeworld of the Kleer Skeletons, the enemies heavily featured in the previous Serious Sam installments. Kleer is a fiery planet filled with lava and active volcanoes. The fourth medallion part is retrieved after Sam defeats Kleerofski, the supreme ruler of Kleer.
Ellenier (populated by Elvians) - A medieval fairy-tale based planet which features beautiful landscapes, towns and castles. The final part of the medallion is located here where Sam acquires it by defeating Cecil, a giant pink dragon who rules over Ellenier. After the battle, Sam spares his life in exchange for the final medallion part.
With the medallion finally complete, Sam is now 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. 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 Sirius' protection shield, which was a last line of defense for the Sirians against Mental. And so, Sam's mission continues.
Kronor - An ice moon where many of Mental's structures and forces are located. Here, Sam has not only to conquer the enemy's base and remove the protection shield of Sirius, but also to lead various people from the planets he visited, who are being held here as prisoners, into the final battle against Mental. After defeating Hugo, a massive robot dispatched by Mental, he activates the cannon that dispatches the shield and soars off to Sirius.
Sirius - A planet that was once populated by the Sirians, now under Mental's control. The traces of Sirians were seen many times by Sam in Earth's past. However, the time of the Sirians' glory was long over, as Mental took over their planet way before Sam's time. Sirius is a high-tech planet full of skyscrapers, energy fields and other stereotypical science fiction scenery. You fight through Siriusopolis Uptown and Downtown before reaching Mental. However, when Sam makes his way to Mental's heavily protected structure, Mental Institution, he has to battle the building itself, which is revealed to be quite a powerful adversary (In the Xbox version, the player doesn't fight this boss).
In the endgame, Sam enters Mental Institution after disabling it and into Mental's throne room. Sam finally encounters Mental in person while the lights are off and while Mental tries to reveal to Sam that he is his father, Sam interrupted by shooting him, thus silencing Mental forever. After that, the Sirian Great Council, even the inhabitants from the planets, celebrate their long-achieving victory against Mental. When the lights come back on however, it is revealed that Mental wasn't there at all and it was instead just a speaker attached to Mental's throne, implying that he actually tricked Sam in order to escape in his starship into deep space (In the Xbox version, the game doesn't show this scene). The credits roll after that, where a dialog of three unknown people is played discussing how big Mental's boss fight could be.
After the credits, a silent-motion scene shows Sam returning to the Sirian Great Council with the medallion in hand. As they take the medallion however, Sam soon discovers that the Council had cardboard-boxes full of those medallions. Sam then becomes enraged and chases the Council around
their room, thus giving the final conclusion to the story.
System requirements
Minimum Requirements:
Microsoft Windows XP
Intel Pentium 4 2 GHz or Pentium M 1.5 GHz or AMD Athlon XP 1.5 GHz
256 MB of RAM
NVIDIA GeForce 3 or ATi Radeon 8500 or S3 DeltaChrome or XGI Volari
3 GB of free hard disk space
Recommended Requirements:
Microsoft Windows XP
Intel Pentium 4 3 GHz or AMD Athlon 64 2 GHz
512 MB of RAM
NVIDIA GeForce 6800 or ATi Radeon X800
3 GB of free hard disk space