The Sentinel

The Sentinel is the twentieth episode of the fifth season of Stargate SG-1.

Plot
The episode begins as General Hammond attempts to contact SG-9 on P2A-018, who are eleven hours late reporting in. SG-9 was attempting to re-establish diplomatic ties with the Latonans, creators of an advanced defensive weapon named the "Sentinel", which was developed over five-hundred years ago by their ancestors, to defend the planet. Contact is established with Sergeant Grogan, a member of SG-9, who reports that SG-9 was attacked by Jaffa and that he is cut off from the Stargate, but is then attacked by a Jaffa patrol and cuts out.

General Hammond briefs SG-1, explaining that two years previously an NID team attempted to analyze the Sentinel, to create copies to defend Earth, but were refused access to the weapon by the Latonans. Immediately after the NID team returned from their mission, they were captured, imprisoned, and placed on death row for stealing technology from Earth's allies. Jack O'Neill volunteers to visit them, as a consensus is reached that the NID team lied and gained access to the Sentinel against the Latonans' wishes and that they somehow damaged the device in the process. O'Neill speaks with Colonel Grieves, who reveals that they disassembled and reassembled the Sentinel in an attempt to understand it. O'Neill asks Grieves to tell him how to fix the Sentinel, but Grieves insists that he and Lieutenant Kershaw, the other member of the NID team, go with them and have their death sentences commuted to life.

SG-1, SG-3 and the two convicts travel to the planet and find Grogan, the only remaining member of SG-9.SG-9 was attacked by the Goa'uld, indicating the Sentinel is inoperative. Major Samantha Carter, Teal'c, Daniel Jackson, Colonel Grieves and Lieutenant Kershaw trek to the Sentinel to repair it while Colonel O'Neill and Lieutenant Grogan set out for the city to offer the Latonans a chance to escape.

Major Carter's team locate the Sentinel and attempt to disable the force field protecting it, an action which took forty-eight hours the first time, according to Kershaw. Teal'c and Carter fight off a Jaffa patrol while Daniel, Grieves and Kershaw attempt to deactivate the force field. Meanwhile, O'Neill and Grogan attempt to persuade Marul, the leader of the Latonans, to save his people and evacuate the population through the Stargate. Jack is unable to convince him, as Marul insists that as long as the candle is his chambers burns, the Sentinel and its caretaker are protecting them. A Goa'uld Ha'tak begins bombarding the city and Jack tries one last time to convince Marul, but is ambushed by Jaffa and captured.

Daniel succeeds in deactivating the force field as a battalion of Jaffa approach, and it is reactivated in just time to protect the group from the Jaffa, but Kershaw is shot in the back with a staff blast as she activates the force field. Grieves attempts to repair the Sentinel while the Jaffa try to shut down the force field, but he cannot find anything wrong. Daniel then translates a small portion of text on the Sentinel, reading 'Life Force', 'Life Energy', and 'Two as one'. Eventually Grieves admits, at Kershaw's behest, that he shot and killed a Latonan man claiming to be the Caretaker, then Kershaw rigged a dead man's switch that the Caretaker was wearing, so that it was not set off when the Caretaker died.

The Jaffa bring Jack and Grogan to the Sentinel and demand that Carter's team lower the force field. Jack orders Carter to destroy the weapon, but Grieves tells the Jaffa he will deactivate the force field from inside. Jack repeats his order and Carter attempts to communicate to him that she can tell Grieves knows how to fix it. Grieves realizes what they are doing incorrectly, and grabs hold of the Sentinel, making "Two as one". The Sentinel releases a beam of bright light and the Jaffa disappear, as does Grieves. When the beam has passed, Carter realizes Kershaw has died from her staff wounds. Teal'c suggests they ask the Latonan for permission to study the Sentinel further, but Jack merely replies that he believes he already knows their answer.