Commander Tucker, while endeavoring to increase to stable speed of Enterprise to Warp 5, becomes comatose when the engines destabilize and explode. Doctor Phlox suggests a radical and controversial procedure to save him - growing a mimetic symbiote as a neurological donor. Archer, concerned with the overarching goal of their mission, gives the order to do so. The symbiote, with a natural life-span of two weeks, is then injected with Tucker's blood and is soon born. Phlox names him "Sim", and as he develops rapidly, Tucker's memories and personality begin to express themselves (including an interest in engineering, and a romantic attraction to Sub-Commander T'Pol).
The window of time approaches when Phlox must perform the transplant, which, according to him, will unfortunately now be fatal to Sim-Trip. Sim, having now 'met' himself in sickbay, reveals knowledge of an experimental procedure that might allow for an extension (presumably by decades) of his lifespan. Archer learns that Phlox was aware of this procedure, but concealed his knowledge of it due to its highly experimental and poorly researched nature. Sim then expresses a strong desire to live —Tucker's own life notwithstanding—through the rest of his natural life. Later, a tired-looking Archer, angered at finding Sim in Trip's quarters, states he would rather Sim voluntarily submit to the fatal procedure than be forced to do so.
Sim eventually resigns himself to the transplant, after contemplating the fate of his sister back on Earth, but not before attempting to escape in a shuttle-pod. He realizes his and the crew's options are limited (given the fact that the ship is stranded without engines in highly magnetic and life-threatening particle field). Before leaving for Sickbay, T'Pol arrives and gives him a kiss. With the medical procedure successful, Archer honors his sacrifice in the presence of part of the crew (including Tucker). The episode then begins and ends with the funeral and burial of Sim in space.