Stargate SG-1 follows the present-day adventures of SG-1, a military team from Earth that, along with two dozen other SG teams, ventures to distant planets through an alien device known as a Stargate which is housed in a top-secret U.S. military base known as the Stargate Command (SGC) underneath Cheyenne Mountain. The mission of the SG teams is to explore the galaxy and search technology and alliances with friendly advanced races for defense from the Goa'uld, a parasitic alien race which started taking humans as unwilling hosts thousands of years ago and transported them to other habitable planets across the galaxy. The series expands upon the Egyptian mythologies presented in the Stargate film, the Norse, Greeks and Romans, but also has contained one-episode stories. The Goa'uld serve as the primary antagonists during the first eight seasons. The Replicators, self-replicating mechanical bugs, are introduced as a recurring villain race in the third season. The Ori, who like the Ancients are Ascended Beings with extraordinary powers, are introduced in season 9 and pose a departure from the primary focus on Egyptian mythology into an exploration of the Arthurian mythology. Many well-known mythical locations such as Avalon, Camelot and Atlantis are found in later seasons.
In the pilot episode, which takes places one year after the events of the original film, the Goa'uld System Lord Apophis (Peter Williams) makes a violent visit to Earth. When the Earth Stargate is revealed to be part of an enormous intergalactic network connecting countless planets, the SGC military base is brought back into action and SG teams are created to help defend Earth from the Goa'uld, who have interstellar pyramid-shaped warships and vast armies of modified humans known as Jaffa at their disposal. The First Prime of Apophis, a Jaffa named Teal'c (Christopher Judge), defects from his master in the pilot episode in the hope that the Earth humans can free his race from Goa'uld rule. He joins Col. Jack O'Neill (Richard Dean Anderson), Dr. Daniel Jackson (Michael Shanks), Cpt. Samantha Carter (Amanda Tapping) and becomes part of the SG-1 team. Maj. Gen. George Hammond (Don S. Davis) serves as the leader of the SGC.
Earth is exposed as a threat to Goa'uld power throughout the seasons and comes under attack multiple times. SG-1 and the SGC forge several alliances with other races in the galaxy, among them the Tok'ra, the Tollan, the Nox, the Asgard, and the remnants of an extinct advanced race that comes to be known as the Ancients, the original builders of the Stargates. Meanwhile, forces on Earth attempt to take control of the Stargate and other alien technology for their own ends, in particular rogue agents of the NID agents. Another alien threat arises in season 3 in the form of non-sentient machines called Replicators, who are on the verge of wiping out the Asgard, one of Earth's greatest allies. Season 4 of Stargate SG-1 ends with a major battle against Apophis.
Position Jack O'Neill (Richard Dean Anderson) Seasons 1–8,recurring in 9 & 10 Colonel (Seasons 1–8), Brigadier General (Season 8),Major General (Seasons 9-10, Stargate: Continuum), U.S.A.F.
Daniel Jackson (Michael Shanks) Seasons 1–5 & 7–10,recurring in 6 Civilian / Archaeologist and Linguistics Expert
Samantha Carter (Amanda Tapping) Seasons 1–10 Captain (Seasons 1–3), Major (Seasons 3–8),Lieutenant Colonel (Seasons 8–10, Stargate: Ark of Truth),Colonel (Atlantis Season 4, Stargate: Continuum), U.S.A.F.
Teal'c (Christopher Judge) Seasons 1–10 Former First Prime of System Lord Apophis,Warrior and respected leader within the Free Jaffa.Does not hold a U.S.A.F. rank.
George Hammond (Don S. Davis) Seasons 1–7,recurring in 8–10 Major General (Seasons 1–7),Lieutenant General (Season 8–9, Stargate: Continuum), U.S.A.F.
Jonas Quinn (Corin Nemec) Season 6,
recurring in 5 & 7 Civilian / Alien Scientist
Cameron Mitchell (Ben Browder) Seasons 9 & 10 Lieutenant Colonel (Seasons 9-10, Stargate: Ark of Truth),
Colonel (Stargate: Continuum), U.S.A.F.
Hank Landry (Beau Bridges) Seasons 9 & 10 Major Genera, U.S.A.F.
Vala Mal Doran (Claudia Black) Season 10,recurring in 8 & 9 Alien/Civilian, ex-Goa'uld Quetesh