“Tabula Rasa” is episode six of season four of Stargate: Atlantis.
At the start of the episode, Rodney McKay comes too tied to a desk, with a note reading ‘Press Here’ on the console in front of him (given the human propensity for pressing buttons, ‘Don’t Press Here’ would probably have worked just as well). There is a message from himself saying that he has to find Teyla and not trust anyone else.
The episode then goes back to 14 hours earlier. People start complaining of headaches and dizziness. It appears that the recent trip to the mainland resulted in a bacteria being brought back to Atlantis. The story then goes back to the present, with most people having lost much of their memories, and then it continues swapping back and forth between the present and the past.
A race against time before the disease kills everyone episode.