 David Marcus ( Merritt Butrick ) — Kirk 's son , a key scientist in Genesis 's development — and Lieutenant Saavik ( Robin Curtis ) are investigating the Genesis planet on board the science vessel Grissom . Discovering an unexpected life form on the surface , Marcus and Saavik transport to the planet . They find that the Genesis Device has resurrected Spock in the form of a child , although his mind is not present . Marcus admits that he used unstable " protomatter " in the development of the Genesis Device , causing Spock to age rapidly and meaning the planet will be destroyed within hours . Meanwhile , Kruge ( Christopher Lloyd ) , the commander of a Klingon vessel , intercepts information about Genesis . Believing the device to be potentially useful as a weapon , he takes his cloaked ship to the Genesis planet , destroys the Grissom , and takes hostage Marcus , Saavik , and Spock . 
