SG-9 are 11 hours overdue from the planet L'tona (P2A-018). Hammond has the planet dialled up and they try to establish contact. They manage to speak to Lt Grogan who tells them that he can't make it to the stargate and is under fire from staff weapons.

In a briefing Hammond tells them that the NID operation visited L'tona and may have done something to the planetary defence system, the Sentinel. Sam says they need to speak to the two team members who went there, Greaves and Kershaw. They may be the only people who can fix it.

Jack goes to see Greaves who isn't very co-operative, that is until Jack says he will go and speak to Kershaw. Greaves tells Jack that they didn't do anything to the Sentinel but that if they are to go to L'tona he and Kershaw are going as well to fix it, he also wants his conviction commuted.

At the SGC they use their new missile system to eliminate the two staff emplacements guarding the gate, then go through. While SG-3 go after the few remaining Jaffa SG-1 head out to where the Sentinel is. As they head towards the Sentinel they meet up with Grogan. Jack has Grogan take him to Maurul (the L'tonan leader) in the city while Sam leads the others. They arrive at the Sentinel, Greaves and Kershaw start to work on the forcefield protecting the Sentinel.

In the city a Jaffa tells Maurul that his people are ignoring the curfew they have imposed and are not preparing for Lord Svarog's arrival. Maurul tells him that his people will not serve anyone and that the Sentinel will send them away. After the Jaffa have gone, O'Neill and Grogan enter, O'Neill tells him what is going on.

A while later a Jaffa patrol finds Sam and the others at the Sentinel site. Greaves tells Daniel to give him his Beretta because he can help outside with the Jaffa and Daniel can help with the forcefield. They manage to drive the patrol back. Meanwhile a ship opens fire on the city from orbit, Svarog has arrived.

Kershaw and Daniel manage to get the forcefield down and start to work on the Sentinel. Outside more Jaffa arrive, Sam and Teal'c drop back into the Sentinel cave. Kershaw and Daniel are told to get the forcefield back up, they do so but not before Kershaw is hit by a staff blast.

In the city Maurul tells Jack that someone called the Caretaker looks after the Sentinel, it will be okay because he is alive as long as the candle in his room burns. Jack tries to contact Sam to tell her about the Caretaker but can't get through. Jaffa enter the room and capture Jack and Grogan.

Kershaw tells Sam and the others about the Caretaker. It appears that the last time they were here, Greaves killed him when he snuck up on them, Kershaw also rigged the flame to continue to burn. Daniel finds some symbols that say 'lifeforce' and 'two as one', they think the Caretaker was an intergral part of the machine, that is why it isn't working.

Jack and Grogan are brought into the cave by the Jaffa. They tell Sam to lower the forcefield or Jack and Grogan will die. Greaves says he can lower the field, much to Jack's annoyance. He steps over to the Sentinel and touches it, a white light suddenly spreads out from the device and all the Jaffa disappear, but so does Greaves. Kershaw dies from her injuries as well. Teal'c says that the Sentinel is indeed a powerful weapon and that they should ask the L'tonans if they could study it. Jack tells him that he knows what they would say to that request.