Tabula Rasa

S4, E6: Plant samples brought back from a new planet introduce an amnesia-inducing virus that strikes everyone in the city except Teyla (Rachel Luttrell) and Ronon (Jason Momoa) who appear to be immune. Carter (Amanda Tapping) orders the Gate's control crystal removed to stop the disease from spreading to other planets while Major Lorne (Kavan Smith) and his soldiers are given orders to control the Atlantis population with stun guns. The team discovers that a cure might be available among the plants on the mainland so Ronon and Sheppard (Joe Flanigan) whose memory is also fading set off to obtain it. Meanwhile McKay (David Hewlett) works on a widespread distribution method but is unable to complete it before he too succumbs to the virus. When McKay wakes up he is strapped to his desk in front of his console with no idea who he is or what is going on. Following instructions from a video he recorded earlier McKay frees himself and sets out to find Teyla (Rachel Luttrell) the key to unlocking this mystery.