Admiral James T. Kirk finds out that Spock 's essence ( his soul ) is inside Leonard McCoy . They return to Genesis , where Spock died , to find his remains and take them to Vulcan . When James T. Kirk ( William Shatner ) learns that Spock 's spirit , or katra , is held in the mind of Leonard McCoy ( DeForest Kelley ) , Kirk and company steal the Enterprise to return Spock 's body to his home planet .