Tabula Rasa

"Tabula Rasa" is the sixth episode of the fourth season of Stargate Atlantis.

Synopsis
A mysterious illness causes everyone in the Atlantis Expedition to lose their memories, which will soon kill them all. However, Ronon Dex and Teyla Emmagan haven't been infected, and the fate of the city truly rests on their hands as they try to convince the team to save themselves.

Plot
At night in Atlantis, a scared and confused Rodney McKay wakes up sitting on a chair, with one hand tied to an armrest. He asks for help, but there is none. He then finds a note on a computer tablet, telling him to turn it on. On it is a recorded message that shows McKay. He tells him that he was the one who tied himself up, and tells him to grab a knife from under the seat and free himself. He then tells her that it is vitally important to find a woman named Teyla Emmagan, as she is the only hope to save the city, or hundreds will die. He also tells himself not to trust anybody, before showing him the picture and description of Teyla.

Fourteen hours earlier, McKay visits the Botany lab, to see Dr. Katie Brown. She has been busy collecting plant samples from the mainland of M35-117. She then shows him a cactus-like plant, which she wants to name after him, called a Rodneyana villosa. McKay then wonders if she wants to join him lunch. However, she says she feels ill, with a headache. They both visit the infirmary, when Jennifer Keller tells them both to stay, since several people from the Botany department have fallen for the same symptoms. In the control room, Samantha Carter talks with John Sheppard about the Replicator-Wraith war, where they learn the Wraith are now fighting back, as they gathered debris from one or two Replicator ships over M6R-214. They then hear from Keller, who tells them of the situation of the botany lab, where everyone in the infirmary are infected.

In the present, McKay encounters a dead person, and then encounters Radek Zelenka, wielding a metal rod. They evidently don't know one another, and Radek doesn't know who he is and urges McKay to come with him to safety, saying the "soldiers" will shoot them and take them away if they're hanging around. When McKay insists on keeping his search for Teyla, Zelenka storms out back to hiding.

Ten hours earlier, Keller's team are in haz-mat suits, when Marie falls ill, even with the haz-mat suits. This illness has somehow breached quarantine. The entire city is infected. Keller reccomends all Stargate travel in or out of the city suspended. Worse, McKay notices that Katie Brown is starting to loose her memories, when she doesn't remember who McKay is.

Back in the present, McKay is still looking for Teyla, when he is noticed by Evan Lorne and the soldiers. They stun him and take him away, but McKay's computer tablet is left behind, with Teyla's photo shown on it.

Eight hours earlier, Carter confirms that the entire city is infected, and still no quarantine lockdown. She gets a call from Keller, who tells her the prognosis. The illness is being caused by a bacterium that interferes with the infected individual's memory, and that the conditions gets progressively worse without treatment. The severity and timing of the illness varies from person to person. She then admits that she is feeling the headache, and will start to lose her memories at around six hours time. Carter and Sheppard decide to use the mess hall as a secondary ward to evacuate the people to.

In the present, McKay awakens in the mess hall, with several dozen people, including Keller, wondering who he is. McKay notices the computer is missing, and wonders if anybody knows Teyla. They don't. He also tried to open the door, but it would not open.

In the past, Teyla and Ronon Dex ready the mess hall for new patients. They both discuss the fact that they have yet to experience symptoms. Ronon meets with Dr. Keller and tells her that he is not having any headaches. Thinking he has an immunity to it, Keller takes a blood sample from him.

In the present, McKay takes note of everything he remembers from his own message, to prevent him from forgetting. He then announces to everyone that they must escape somehow, since Teyla is their only chance. However, he forgets her name, until Carter jogs his memory. Carter agrees with him and tells everyone that the soldiers most likely don't know anything either, and need to formulate an escape plan.

In the past, McKay is now out and about, since there is no point in keeping with the infirmary. Both he and Sheppard both experience headaches, so it is only a matter of time before they start to lose memories. Carter tells them the situation, and gives them stimulants from Dr. Nieves, who says will slow down the effects of the memory loss. Carter also says that she ordered Zelenka to remove the Control crystals to the gate, and is hopeful Keller will find a solution before everyone will lose their memories. Meanwhile, Ronon checks up on Keller's progress on him, only to discover that she has started to lose her memories, which hampers her ability to treat the patients. Suddenly, one of the patients begins to convulse and, despite Keller's best efforts, ultimately dies.

In the present, Lorne and the team drop off more loose members and opens the door, when the crew ambushes the soldiers. Lorne and his team stun as many as they can. Keller points Carter and McKay to a hallway, only to get stunned herself. Lorne gives chase to Carter and McKay, but soon looses them. As they rest, Carter wants to know where Teyla is, and agree on searching room by room, thinking the complex isn't that big, until they open a door to the outside, realizing they have a city to explore.

In the past, Sheppard and Ronon escort a confused group to the mess hall, and already seem to encounter resistance. He tells Carter that he ordered Lorne's team to arm themselves with stunners. Sheppard, who now starts to lose his memories has an idea, and brings up that idea to Lorne, who is taking a little too many stimulants, which he says should delay the memory loss as much as possible.

In the present, Carter and McKay are lost, but then encounters a paranoid Zelenka again, this time armed with a stunner. McKay knows him and notices that he is holding the computer tablet McKay lost. In it is the picture of Teyla, saying "find her", but have no idea how.

In the past, Keller has found the mysterious illness, but no longer looked through the Atlantis database, but looked through Carson Beckett's records of a common Pegasus Galaxy form of a childhood disease called Kirsen fever, an illness that Teyla had at only eight, and Ronon at ten. McKay theorizes that the illness was transported to M35-117 by the Ancients when they scouted it, where it then mutated separately from the rest of the Pegasus Galaxy version, as the planet they're on had no Stargate before Atlantis arrived. The illness now adversely affects humans not native to the Pegasus Galaxy, as most natives have an immunity from previously having it when they were children. This is why Ronon and Teyla are not affected, because they had it and grew an immunity to it, so they still have antibodies in their systems from that which protects them. Unfortunately it would take months to synthesize a cure, if it even is possible.

Fortunately Teyla identifies a solution; the Athosians use the Enchuri plant, a plant abundant on many worlds, which was used to treat the illness. Teyla plans to get some from New Athos, but they forget that Zelenka has the gate crystals. They contact Lorne, who tells them that he has seen Zelenka, but he panicked when seeing soldiers and attacked Lorne's team before fleeing. Without the gate crystal, the crew decides to check the mainland for the plant. Ronon talks Sheppard into how to fly a Puddle Jumper to the mainland.

Meanwhile, McKay is working on how to tweak the ventilation system to aerosolise the enchuri plant, when he gets a big headache for a few seconds, and recovers, only to forget what he is doing. Teyla attempts to calm him down and to keep working.

In the present, Carter, McKay and Zelenka continue to elude the patrols by hiding in the ransacked infirmary, when Lorne's soldiers enter. They find what they're looking for, stimulants. It is revealed that in order to stave off the illness, Lorne and his men have been taking those stimulants. While they helped keep their symptoms from worsening, the drugs also served to make the soldiers more paranoid and irrational.

In the past, The Jumper arrives on the mainland, where Sheppard now loses his memory, and gets suspicious of Ronon, who is eventually forced to stun him and tie him up, not wanting him to get in the way. Ronon goes to search for the enchuri plant. Meanwhile, Rodney continues working, and is almost finished, when Teyla hears a noise. She leaves the lab and sees Lorne and a team of soldiers. Lorne insists that Teyla go to the mess hall, ignoring Teyla's plea that she remains to help stop the disease. When Lorne doesn't listen, Teyla knocks out the men and attempts to run to the nearest transporter, but is stunned and taken to holding. Without Teyla, McKay is unable to focus on his job and in desperation, begins recording a message for himself in the future.

In the present, Lorne goes to the city's brig where Teyla is being held. He interrogates her, believing her to be responsible for the illness. Teyla notices Lorne taking the pills, believing them to be responsible for his behavior. McKay, Carter, and Zelenka then arrive and stun Lorne. They say they couldn't locate her on their own, so they followed Lorne. McKay had written the code to the force-field on his arm, and is able to free Teyla.

At dusk, Ronon returns to the Jumper with the plant, but Sheppard is not where Ronon left him. Ronon looks for him, but is ambushed by the amnesiac Sheppard, who holds him at gunpoint. Ronon manages to talk Sheppard down and convince him that he is a friend. Sheppard is convinced to lower his gun and board the Jumper to return to the city. Meanwhile, Carter, McKay, Teyla and Zelenka return to McKay's lab, where McKay completes the preparations to disperse the cure throughout the city. It turns out, that when he said he was almost done, all he had left to do was press "Enter". They then have to wait for Ronon and Sheppard.

In the control room, the soldiers detect the oncoming craft and confront Sheppard and Ronon. Ronon tells Lorne, who woke up and has returned to look in his vest pocket. There, he finds a picture of Sheppard with writing that tells Lorne that Sheppard is the commanding officer and to obey his orders. This was Sheppard and Lorne's backup plan in case the both of them lost their memories. Lorne defers to Sheppard and Ronon's authority and they go to deliver the cure.

One day later, Sheppard wakes up in the infirmary. The cure was spread through the city's ventilation systems and everybody has regained their memory in time to save most of the crew from the fatal stages of the illness. Meanwhile, McKay has been at Katie's side ever since the cure had been distributed, slept by her side and refusing to leave, even after Keller's assurances that she will tell him when she'll wake. Later, Katie wakes up and remembers McKay, which makes both of them glad.

Notable quotes
Sheppard: You showing any signs? McKay: Headache. Sheppard: Yeah, same here. McKay: Five bucks says you start losing your memories first. Sheppard: Rodney!

McKay: I once thought it was Monday five days in a row.

After a forgetful Lorne gets a note saying Sheppard is their leader Sheppard: That's right. I am your commanding officer, so you should do what I say. Lorne: Yes, sir. What are your orders? Sheppard: (Does not know, points to Ronon) Do what he says.

Background notes

 * The notes written on McKay's arm when he was forgetting his memories include the code to unlock the door to the brig, as well as "little guy with glasses" (Zelenka), "avoid soldiers", "100s will die", "Teyla" and "tablet computer".
 * The title of this episode is Latin, meaning "erased tablet." A similar phrase is "blank slate".  In other words, a fresh start without history.
 * Parts of this episode, especially the opening scene with McKay getting his bearings through messages he left for himself and writing on his body, and Lorne's note to himself written on a Polaroid photograph, are very reminiscent of the movie Memento.
 * The name of the nurse played by Linda Ko is revealed as Marie.

Production

 * Zach Selwyn, from G4's Attack of the Show, has a cameo in "Tabula Rasa" as a scientist. Selwyn was invited to the show after he created a rap video for his television show about wanting to be on Stargate Atlantis.
 * Niall Matter guest stars as Lt. Kemp. Matter is better known for his role as Zane Donovan on Eureka, another popular series on the Sci Fi Channel.