[5][6] In the single-player campaign, the player assumes the role of hero Sam "Serious" Stone in his adventures against the forces of the extraterrestrial overlord, "Mental", who seeks to destroy humanity.

He is guided by the Sirian Great Council and receives sporadic aid from the natives of the worlds he visits.

Serious Sam 2's gameplay consists almost entirely of the player attempting to defeat dozens of enemies at a time, and thus is relatively simple.

[9][10] The story is developed through the use of cut scenes, which are interspersed throughout the game, especially at the beginning and end of each Planet.

More complicated gameplay mechanics that are often found in other games (such as jumping puzzles) are rare, and when encountered they are fairly simple, usually requiring the player to locate keys/objects in order to unlock doors or advance to the next level.

[9][10][13] Blood and gore effects have been improved relative to the previous games, and all enemies other than bosses can be gibbed.

[14] Living foes can disintegrate into blood and bloody bits, undead entities, excluding Kleer Skeletons, can be reduced to decaying bits and slime, while magical creatures' destruction is marked with sparkle effects and purple gases.

The player is able to pick up certain objects and manipulate them in a manner similar to the effects of the Gravity Gun from Half-Life 2, though without the necessity of an external device.

There are five different groups of NPCs in the game: "Sirians", "Simbas", "Zixies", "Chi Che", and "Elvians", each native to their respective planet.

A prominent feature in the previous Serious Sam games was cooperative gameplay, in which multiple players could play the single-player campaign together.

The minigun, a staple of the series, also makes a return, and is a weapon of particular significance as it was featured prominently on the cover of the box for many of the previous Serious Sam games.

The Serious Bomb also made a return, maintaining its status as the most powerful weapon in the game by being able to eliminate every enemy on the screen.

The Serious Bomb is described as a "miniature big bang" and as "Instant Death With a Smile," and the player is only able to carry a maximum of three due to their size.

Serious Sam 2 introduced new weapons to the series, including "Clawdovic Cacadoos Vulgaris," (the name is pronounced in Croatian as Klodovik, which is a reference to a parrot Klodovik in the comic series Alan Ford ) a parrot clutching a bomb in its talons that can fly to an enemy to eliminate it, and throwable hand grenades.

The game begins with Sam being summoned before the Sirian Great Council, where they provide him with guidance on how to accomplish his goal to defeat Mental.

The credits roll after that, where a dialog of three unknown people is played discussing how this ending might just be another one of Croteam's bad jokes and how big Mental's real boss fight could be.

After the credits, a silent-motion scene shows Sam returning to the Sirian Great Council and handing over the medallion.

[19] For a period of time, Croteam posted weekly updates, but these became bi-monthly, then monthly, and then stopped completely after August 2004,[20] as Gathering of Developers shut down and folded into 2K Games, Croteam's publisher, wanted to handle the release of information from that point forward.

A result of the E3 presentation was a 30-minute video preview of the game featuring gameplay footage while a question and answer session took place between fansite Seriously!

[36] Its highest mark by mainstream media was a 4.5/5.0 from Computer Gaming World, though most reviews were in the 70% or 80% range.

[41] An over-emphasis on reflective surfaces and oddly out of place pixel shading were perceived as being more about showing off the capabilities of the engine than about defining the atmosphere of the game.