Sam, Mitchell, and Vala greet Teal'c as he returns through the Stargate and reports that the Ori have conquered six more worlds under the leadership of Vala's daughter. The team speculates that the Ori are leery of the Antarctica base. Daniel interrupts them to reveal he's determined the gate address for the planet with the Holy Grail.
At the briefing Daniel reveals he narrowed down the search for the three possible planets and found a volume in Merlin's library with a picture of a sword with a Gate address engraved. Landry orders them on the mission but tells Vala to stay there when she leaps at the chance to go with them.
Daniel talks to Landry about letting Vala go with them and suggests the general influence the Pentagon. Landry considers it but notes that he'll be counting on Daniel to keep her in line. Daniel is suitable leery. Later he tells Vala she'll have to undergo a psych exam and she's confident she can pass any test.
SG-1 arrives on the planet and finds a deserted town. Landry visits Vala and finds her hard at work rehearsing her answers for the examination. SG-1 enters a building and finds skeletal corpses. Daniel checks the village records and finds a reference to Morgan le Fay and a cave. He figures she took the weapon for Merlin and hid it. SG-1 calls in a medical team and then searches the woods, noticing that there is no animal life. They search one cavern along with one of the medical team, Ackerman, who displays abrupt drowsiness. They don't find anything in that cavern and move on to the next.
Vala meets with Dr. Hutchison and immediately tries to impress him with her knowledge of psychiatric terms. After she tosses out a series of rehearsed answers for the Rorschach test, Hutchison begins to suspect something is up.
The search teams return for the night and they head for the Gate, only to find Ackerman in a deep unbreakable sleep. Dr. Reimer, the head of the medical team, doesn't have any answers but confirms everyone is infected and they can't go back. He recommends they must stay awake--if they fall asleep they may never wake.
Landry sends the medical equipment through but agrees that they can't risk another team, even in hazmat clothing. They conclude that the disease is also making them sleepy, and noticed heightened levels of melatonin. Daniel finds records saying that Morgan le Fay cursed them with a sleeping sickness. They go to the cavern to get soil samples.
Vala goes to Landry for the results and he tells her she'll be undergoing more examination. He suggests she takes things seriously and doesn't buy her claim that she is. She's preparing in her quarters when Agent Woolsey visits and offers to help her pass her test in return for her providing inside information to him. When she questions him he notes that she's done plenty of shady things in the past.
Sam has located an energy spike and finds a concealed cavern. Teal'c and Mitchell go in to explore while Sam takes the samples back.
Hutchison hooks Vala up to a lie detector test and first asks her some control questions, which she has trouble answering truthfully.
Dr. Reimer finds a dormant parasite in the soil samples and determines that it activates when it touches living tissue. Daniel comes in and they realize one of the medical team is missing. Mitchell and Teal'c move through the cavern and hear a noise, while Sam and Daniel find the missing man asleep. When they come back, Reimer reveals that Ackerman is dead. Performing an autopsy they find he died of an aneurism caused by an enlarged parasite.
Vala returns to her quarters to find Woolsey waiting for her. She feels the questions reminded her of why she doesn't belong there, and reveals she's come to a decision about Woolsey's offer.
Mitchell and Teal'c find a lizard and wonder if it has an immunity to the parasites. They can't radio back from within the caverns so starting hunting the lizard down to capture it. Reimer and Sam determine the parasite made its way to Ackerman's brain and then fed on the melatonin until it grew large enough to cause the aneurism. They're interrupted when Reimer has a heart attack brought on by the stimulants they've been taking. They try to revive him without success.
Hutchison confronts Vala who feels he's already made up his mind about her. She goes on about how she's been a thief and may still steal no matter what, but she's recently been reconsidering her behavior now that she's found someone who believes in her for herself.
They can no longer take stimulants so Sam and Daniel have to rely on each other to stay awake. Teal'c and Mitchell are on their last legs but manage to capture the lizard. Mitchell is in too bad a shape to go back and Teal'c has to leave without him. Daniel and Sam are about ready to pass out when a hazmat team arrives, gets them into an isolation pod, and goes after Teal'c and Mitchell. Sam tries to warn them they can't go to sleep, without success.
As Teal'c makes his way through the caves, the hazmat team move Daniel and Sam. Teal'c gets to the surface and collapses as the hazmat team finds him. They take the animal and create a cure, and later the team is back to normal and ready to go. Daniel mentions he found a reference to Atlantis and suspects the answer may lie in their database.
They're interrupted by a call to the gateroom. Vala demands they let her go through the gate but Landry reveals that she passed the examination and Hutchison thinks she has "potential." Woolsey shows up to reveal that his offer was part of the test. Mitchell warns her that she's only a probation SGC member and warns her not to screw up… but welcomes her aboard. She's eager to start going off-world so she can start getting loot...