David Marcus was a scientist working for the Federation in the latter half of the 23rd century. The son of James Kirk and Carol Marcus, he grew up with his mother after his parents' separation and collaborated with her on the Genesis project.
Work on Genesis
Marcus worked on the Regula space station when it was attacked by Khan. Fleeing with his mother to the Genesis Cave test site within the Regula planetoid, he was rescued by Kirk and brought aboard the Enterprise, where he and his father tried to mend their strained relationship. He was instrumental in identifying the Genesis wave set off aboard the Reliant by Khan.
Following the accidental creation of the Genesis planet, Marcus visited the new world with lieutenant Saavik, taking custody of the rejuvenated Spock. After his capture by Kruge's forces, he sacrificed himself to save Saavik's life, dying from a knife wound received during a struggle with his Klingon captors.
His loss would haunt Kirk for years to come, and the anger he felt over this death shaped Kirk's attitudes towards the Klingons leading up to the Khitomer negotiations.