Tucker falls into a coma, and Phlox creates a rapidly aging clone of him to harvest tissue that could save his life.
While Tucker tries to push Enterprise's stable top speed to warp five, the engines explode and leave him in a coma. Phlox proposes growing a mimetic symbiote from Tucker's cells to supply neural tissue, and Archer, putting the Xindi mission first, approves. The clone, called Sim, lives only about two weeks, aging rapidly while inheriting Tucker's memories, his love of engineering and his attraction to T'Pol.
As the operation nears, it becomes clear the procedure will kill Sim. He learns of an untested treatment that might extend his life, which Phlox had kept quiet, and briefly plans an escape, but memories of his sister's death in the Xindi attack lead him to accept his role. T'Pol kisses him and he thanks Phlox for being a father to him. The surgery saves Tucker, and the episode opens and closes with Sim's funeral. Manny Coto wrote this, his first script for the series, directed by LeVar Burton.