There But for the Grace of God

There But for the Grace of God is the nineteenth episode of the first season of Stargate SG-1.

Plot
SG-1 is exploring an alien complex on the planet P3R-233. Dr. Daniel Jackson stumbles onto an alien artifact resembling a mirror. He touches the artifact and unknowingly enters an alternate reality. Finding the complex suddenly abandoned, Daniel concludes that the rest of SG-1 returned through the Stargate without him, and dials home.

Upon his return to Earth, Daniel discovers that the SGC has suddenly become quite different, not least of which is the fact that nobody knows who he is. He is arrested by Colonel Hammond and is held captive. Catherine Langford, in charge of the gate program, confronts him and demands to know how he acquired SG-1's code to open the iris. Daniel, still believing that he is in his own reality, insists that he is a member of SG-1. Through his encounter with Catherine, Daniel quickly discovers several differences in this reality, including the fact that he turned down the offer to work on the Stargate program and that Samantha Carter is a civilian astrophysicist assisting the Stargate Association instead of a captain in the US Air Force.

When Daniel tells Catherine that she was retired and living with Ernest Littlefield she is shocked and confronts General Jack O'Neill, who is now commanding officer of the SGA. Daniel uses his probing knowledge of O'Neill, such as his knowledge that O'Neill was contemplating suicide after the death of his son, to get his attention. When Dr. Carter enters, she explains that they "lost" Washington and Philadelphia. Asking what they mean by "lost", Dr. Jackson learns that Earth is under attack by the Goa'uld, One and a half billion people are dead, and most of the world's major population centers have been destroyed; including the Egyptian city that was the last known residence of Daniel's counterpart.

Dr. Carter soon concludes that Daniel is from an alternate reality, triggered by the "quantum mirror". Analyzing a transmission received from the planet he returned from, Daniel, Carter and Catherine realize that the transmission is actually a Stargate address, apparently for the source of the current attack on Earth. O'Neill wants to send a bomb through the 'gate, but Daniel points out that, while this won't stop the problem here, his Earth still has a chance. Despite the fact that he will be the only person they can save, as they lack the time to dial the gate more than once, the alternate SG-1 agree to help Daniel. However, Apophis's Jaffa, led by Teal'c, who in this reality never met SG-1, invade the SGA in retaliation for the destruction of Chulak, making it even more difficult; they have around twenty minutes before the Stargate can be dialed again, and it may be impossible to hold the Jaffa back for that long. Fortunately, Daniel offers O'Neill a video recording of his mission back in his reality, suggesting he show it to Teal'c to buy them time. Before O'Neill confronts Teal'c, Carter fondly embraces him. Catherine explains to a confused Daniel, "I take it they're not engaged in your reality."

One by one, the alternate members of SG-1 die as they try to buy Daniel time to escape; O'Neill is killed by Teal'c, whose wife and son were unfortunately killed in the attack of Chulak, Carter triggers a grenade that kills her and several Jaffa, and Catherine is shot after programming the Stargate for Daniel. The SGA is destroyed to prevent it from falling into Apophis's hands just after Daniel manages to escape through the Stargate, although he is shot by the alternate Teal'c just as he dives through. Daniel barely escapes back to the planet with the mirror, armed with the gate address of the world that is the staging point for the invasion, and with the hope that what he saw in the alternate reality does not come to pass for his Earth.

Goofs

 * Continuity: Daniel puts his video recorder on the table in the "lab" so he can take off his back pack. He then starts stuffing artifacts into his backpack. Before he picks up the second artifact, we can see the recorder on the table, along with another artifact. The camera pans up to Daniel stuffing his backpack, then back to the table where the artifact and the recorder are gone.
 * Continuity: Catherine shows Daniel a world map on which cities destroyed by the Goa'uld are marked by red dots. In a close-up of the map there are red dots in northern and central Europe. In other shots these dots are not present.
 * Continuity: While dialing the gate on the alternate Earth to send a nuke to Chulak, Walter Harriman announces "Chevron Three encoded!", but his lips are clearly not moving.
 * Continuity: While dialing the gate in an attempt to reach the Beta Site, Walter Harriman announces "Chevron Five encoded!", but 13 seconds later, a close-up of the dialing computer shows Chevron Five being encoded.