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"Observer Effect" is the eleventh episode of the fourth season of Star Trek: Enterprise.
As Enterprise orbits a nearby planet, it seems that Travis Mayweather and Malcolm Reed are playing chess in the mess hall. In reality, however, they are under the control of two noncorporeal aliens. They talk about the game in technical terms and discuss the Humans they are observing on Enterprise. When "Mayweather" asks "Reed" how the Humans will react to what they find on the planet's surface, "Reed" answers that, in his experience, at least one member of every species that has come here has died as a result. He also mentions that some species have lost an entire ship's crew.
In a shuttlepod, Commander Trip Tucker and Ensign Hoshi Sato return to Enterprise from the M-class planet, where they were excavating a Klingon garbage site. Tucker hails the starship and speaks with Captain Archer. He tells the captain that their search was unsuccessful. All they found was an old power cell, some empty ration packs and a latrine. Suddenly, Tucker starts coughing uncontrollably and eventually collapses. Sato informs the senior officers of Tucker's condition. "Mayweather" and "Reed" watch as Archer asks Sato to pilot the shuttlepod into the launch bay and requests that T'Pol notify Doctor Phlox to be ready...