Stargate: Atlantis Episode 5.06 The Shrine

En route to check on a missing research team, Lt. Col. Sheppard, Dr. McKay, Teyla and Ronon emerge from the stargate to find themselves swimming atop a flooded valley. With only the apex of the gate accessible above the waterline they cling to it, soaked and bitterly cold, until a puddle jumper can make it through to rescue them.

But during the flight back to Atlantis, McKay inexplicably falls ill and in the days that follow, his condition declines further. Dr. Keller discovers a parasitic organism in his brain that cannot be removed. As a result, his ability to function continues to deteriorate to the point where he has virtually no recollection of who he is or what he is capable of. Soon, his emotions and memories have regressed to those of a young boy and, fearing that he will not pull through, the doctor summons Jeannie, McKay's sister, to say her final goodbyes.

But it's too late. Rodney no longer recognizes her or any of his friends. However, Ronon has a plan, albeit a very risky one. He tells Jeannie of a place, a shrine located on the planet Talus, that would grant Rodney one day to be himself again.