During a routine geological survey on Surata IV, Commander Riker's thigh is pierced by a small thorn, which causes him to lose the feeling in his leg. After beaming Riker back aboard the U.S.S. Enterprise, Dr. Pulaski determines that the thorn contains an unknown organism which is rapidly spreading throughout his nervous system. If left unchecked, the microbe will soon reach Riker's brain and kill him. Ironically, Pulaski has no way of decimating the organism without destroying the nerves of the being it's inhabiting. At first, Riker's affliction seems more of a nuisance than a threat. But during a visit with Counselor Troi, his vital signs suddenly plummet and Riker loses consciousness. Although Dr. Pulaski is able to stabilize his condition, she discovers that the organism has spread to Riker's brain. In a desperate attempt to keep his brain functioning, Pulaski begins electronically stimulating his memory.
