The death of Admiral James Kirk's son David Marcus served a deeper purpose for Star Trek III: The Search For Spock. Spock sacrificed himself to save the Enterprise crew in Star Trek II: The Wrath Of Khan - a loss so profound Kirk had trouble accepting it. However, just before his death Spock transferred his katra - essentially his soul - into Dr. McCoy, meaning he could potentially be resurrected. Kirk and his crew stole the Enterprise out of spacedock to return to the Genesis planet, where they hoped to retrieve Spock's body and reunite it with his katra.
Earlier in The Wrath Of Khan, Kirk met his adult son, David Marcus, for the first time. David helped develop the Genesis project with his mother, Carol - an old flame of Kirk's. During The Search For Spock, David was researching the newly created Genesis planet with Lieutenant Saavik when they encountered a feral, rapidly aging Spock. David and Saavik then ran afoul of a group of renegade Klingons attempting to learn the secrets of the Genesis project. The Enterprise arrived and confronted the Klingons, though they had already captured David, Saavik, and Spock. The Klingon Commander Kruge executed David to force the Enterprise to surrender, pushing Kirk to extreme measures.
The Real Meaning Behind Kirk's Son David's Death in Star Trek III
The Search For Spock would see Kirk endure three massive losses to resurrect his friend. He essentially ended his career by disobeying orders and stealing Star Trek's Enterprise. He then lost the son he was just getting to know at the hands of the Klingons. The final sacrifice was the Enterprise itself. After David's death, Kirk agreed to surrender and the Klingons prepared to board the Enterprise. Kirk and crew activated the Enterprise's self-destruct sequence and transported to the surface of the Genesis planet. With the majority of Kruge's men aboard, the Enterprise exploded, burning up in the atmosphere of the Genesis planet as Kirk looked on in despair.
Kirk was able to kill Kruge in hand-to-hand combat just as the unstable Genesis planet was beginning to tear itself apart. Kirk and his crew commandeered Kruge's Klingon Bird Of Prey and escaped with Spock's body. They met Spock's father Sarek on Vulcan, where Spock's body and mind were finally reunited. Sarek pointed out the horrible sacrifices Kirk had to endure to revive Spock, implicitly asking him if it was all worth it. A physically and spiritually battered Kirk confirmed it was, as had he not tried, the cost would have been his soul.
Kirk's Sacrifices Reinforce His Unbreakable Bond With Spock
In many ways, The Search For Spock is the story that solidified the depth of the relationship between Kirk and Spock. The pair were of course close friends in the days of Star Trek: The Original Series, but by the era of The Search For Spock that friendship had evolved into something approaching familial love. Kirk would actually refer to Spock as his brother in the Star Trek V movie. While the stoic Vulcan was incapable of expressing himself the same way, it was clear Spock felt the same way about Kirk.
Kirk would eventually regain his career and ship after the events of Star Trek IV: The Voyage Home, but the loss of David lingered. Kirk's bitterness over David's death would curdle into full-blown xenophobia toward the Klingons by the time of Star Trek VI: The Undiscovered Country. With Spock's help and David's idealism in mind, Kirk was eventually able to overcome that bigotry and help foster peace between the Federation and the Klingons. Star Trek III: The Search For Spock may have been the darkest chapter in Kirk's life, but they also certainly underscore how important his bond with Spock truly was.