S2E22 · Shades of Gray

During a geological survey on Surata IV, Commander William Riker is struck by a thorn growing on a motile vine plant. The away team immediately beams back to the Enterprise, where Dr. Katherine Pulaski finds out that the thorn has released a deadly virus into Riker's body. Within a matter of hours, the virus will reach Riker's brain, killing him. To try to save Riker's life, Pulaski puts him into a machine that will artificially stimulate his brain neurons, keeping them active and resisting the virus. This causes Riker to dream of his past adventures aboard the Enterprise.

Riker's first dreams are of reasonably neutral occasions, such as his first meeting with Lieutenant Commander Data. He soon moves on to more passionate and even erotic dreams, such as meeting the cheerful young Edo women on Rubicon III, the matriarch Beata on Angel One, and the computer-generated holodeck woman Minuet (Carolyn McCormick from " 11001001 ").

However, while pleasing to Riker's mind, the passionate dreams only worsen Riker's condition, as the virus feeds on the positive endorphins his brain is creating. Pulaski and Counselor Deanna Troi therefore agree to try to make the machine evoke negative dreams instead. Thus Riker dreams of Lieutenant Tasha Yar 's death, and the apparent death of Deanna Troi's child. This has the desired effect, as the negative endorphins drive the virus away, but the endorphins are not strong enough. As a last resort, Pulaski uses the machine to evoke dreams of raw, primitive feelings of fear and survival. Thus Riker dreams of fighting the tar creature Armus, the alien-controlled Admiral Gregory Quinn, and the Klingon officer Klag on board the warship Pagh. Seeing that the raw emotions work best, Pulaski intensifies the dreams to come at a more rapid pace. This finally kills the virus and Riker recovers.