Shades of Gray

During a survey of Surata IV, Riker is pricked by a thorn from a moving vine. A microorganism enters his bloodstream and begins advancing on his brain, and Pulaski has only hours to stop it. She connects him to a device that stimulates his memories, hoping his own nervous system can fight the infection. Riker relives moments from his time aboard the Enterprise. At first the pleasant memories, mostly romantic, only feed the microbe and speed it on. Pulaski and Troi switch to sad memories, including the death of Tasha Yar and the loss of Troi's child, but these merely slow it down. Finally they flood his mind with memories of danger, combat and fear. The resulting surge of aggressive brain chemistry kills the infection, and Riker wakes up cured, though exhausted by the experience.