Doppelganger

S4 E4: While Lt. Col. Sheppard (Joe Flanigan) and the team are out exploring a jungle planet they discover a glowing crystalline growth. Compelled to touch the strange crystal Sheppard becomes host to an alien entity. Once on Atlantis the entity begins to move from person to person through the power of touch. Teyla (Rachel Luttrell) is the first to experience a nightmare involving Sheppard. The chain reaction continues when she infects Dr. Keller (Jewel Staite) who succumbs to a different but equally terrifying dream. When Lorne (Kavan Smith) appears to have a psychotic breakdown the team urgently attempts to trace the problem to its source. When Rodney McKay (David Hewlett) becomes the latest host the team links Sheppard to McKay's brain where Sheppard ultimately faces off against the malevolent entity that has assumed his form.