Serious Sam 1 is the first two games in the Serious Sam series; Serious Sam: The First Encounter and Serious Sam: The Second Encounter. These games are combined into one in the Xbox version of Serious Sam 1, showing that they are intended to be one game.
Serious Sam 1 takes place across four locations; Ancient Egypt, Mesoamerica, Babylon and Medieval Poland and features large-scale battles against many enemies. The levels are linear and cannot be traveled back to once completed, but there are secrets hidden throughout levels, which either contain weapons and ammo, a power up, more enemies, something funny, or a combination of them. Most levels are broken up into a distinct pattern; large arena featuring many monsters and items to signify the end of a wave, then a “break” room or area that has items and/or smaller amounts of enemies for the player to catch their breath in while still being challenged and having fun. They then return to an arena, and it continues until the level ends.
SS1 features a scoring system that gives you points for each enemy you kill. The points given usually depends on the strength and difficulty of an enemy killed; the Beheaded Rocketeer, the weakest and easiest enemy to kill in the game, gives away only 100 points when killed, while the Major Biomechanoid, a large enemy with a lot of health and rockets that can kill someone with 100 or less health in one shot if their rockets directly impact the player, will give the player 8,000 points when killed. There are also bonuses for completing a level under the par amount of time listed in the statistics screen.
Hints and information about weapons and enemies are given to the player via NETRISCA, a computer built into Sam's skull. The game will occasionally have a message pop up, usually when a new area is entered, a new enemy is killed or a new weapon is picked up, for the player to read the computer's analysis of the new thing. Important tips are often contained in messages from NETRICSA (such as what to do in a room), so they player should consult it often.
Weapons are varied, but never become truly obsolete. The Schofield .45 works well against weak enemies because of its low power, decent rate of fire, and infinite ammo; the XPML21 Rocket Launcher is devastating against enemies at medium range and enemies with a lot of health, the XM-214-A Mini-gun is useful against everything because of its extremely high rate of fire, while the SBC Cannon is the ultimate weapon against multiple enemies, because it can pierce multiple enemies while traveling, and against strong enemies because its projectiles can be charged up.
Various items and power ups are scattered throughout levels. Various types of armor appear throughout levels, which absorb a certain amount of damage from enemy attacks when picked up, backpacks filled with ammo are usually placed near areas where there's going to be a huge fight, and various health pickups are scattered throughout levels for the player to recover lost health.
SS1's primary offline game mode is the single player campaign. The player must travel through Ancient Egypt-themed levels while fighting off enemies. After defeating the final boss, they win. Offline also features split-screen, which lets up to four people play the game's multiplayer modes in one PC.
Online contains more content. The most prominent online game mode is cooperative, which lets up to 16 people play through the campaign online. Functions such as the amount of respawns, enemy health and extra co-op enemies can be modified before starting a game. There are also two competitive modes; death-match and score-match. Death-match is the usual Quake-style death-match, but Score-match is different; everything the player does in a Score-match (pick up items, kill someone, etc) will give them points. However, they will lose points if they are killed. The person that either reaches the score limit are has the most points at the end of the time limit wins. These can also be played in offline mode's split-screen mode.
In the near future, humanity discovers very advanced technology in Egypt that allows humans to travel through deep space with ease. These items are from the Sirians, an extinct species that were quite similar to humans. However, they were wiped out by a strange creature called Chaad Sheen, who apparently appeared in the universe in order to wipe out all intelligent life every once in a while. Humans ignore the Sirian's warnings about Chaad Sheen and travel through the universe on technology based on a Sirian spaceship parked on one of Jupiter's moons; the USS Centerpiece.
While exploring Sirius, Serious Sam discovers a large asteroid belt surrounding it that wasn't mentioned in the old Sirian records. He decided to go through it, which is actually a large alarm system for Chaad Sheen, because he wasn't sure that he killed ALL of the intelligent life in the universe back when he killed the Sirians, so he left the alarm system just in case. After being woken up, Chaad Sheen manages to defeat the humans, one battle after another. His power and craziness earns him the name Mental. One of the heroes during the battles is Serious Sam, who constantly and ruthlessly fights off every alien he encounters.
After many battles, humanity is doomed. Mental's forces are invading Earth; humanity's last stronghold, and there's no way to stop him. One of the scientists points out that a time-travelling device called the Time-Lock was found in the Sirian ruins. It turns out the machine was last set back in Ancient Egypt, which would be when Mental was finishing off the Sirians. A plan is devised; using the Time-Lock, someone goes back in time, summons the USS Centerpiece, flies to parts of Sirius' old trading routes, then uses the ships there to sneak up on and fight Mental while he's busy fighting off the Sirians. He won't expect someone to fight him one on one while he's busy doing something else. Its a desperate plan, but humanity is all out of options. The person chosen to go through the Time-Lock is Serious Sam because of how he can easily go against hordes of monsters and survive.
However, as Serious Sam 3: BFE shows, getting to the Time-Lock isn't as easy as planned...
The first part of Serious Sam 1', The First Encounter, is about Sam traveling through Ancient Egypt in order to find a spaceship so that he can travel to Sirius and kill Mental. The Second Encounter is about Sam having to find a backup ship because the one he got before was damaged and fell back to Earth.