While exploring a then-unexplored planet, Surata Four, Riker gets jabbed in the calf, and is infected by mysterious microbes which attach themselves to nerve endings and prevent neural activity. Riker's leg soon goes dead, and Pulaski finds that the infection is spreading, and when it reaches the brain, Riker will die.

A few hours later, Riker slips into a coma, and Pulaski decides to temporize by externally inducing neural activity. She finds he is reliving old memories, and with Troi's help, figures out exactly which emotion he's feeling at the time. Furthermore, each emotion is releasing different brain endorphins, some of which may well be poisonous to the organism.

After determining that sexual arousal merely stimulates the organism's growth, she and Troi find that strong negative emotions drive the organism off. By tightening the focus of the beam, she induces the most basic, primal, "survival emotions", and Riker recovers, weak but intact.