Sarek's appearance on Star Trek: The Next Generation was the first major crossover of an Original Series character onto the new show, other than McCoy's cameo appearance in "Encounter at Farpoint" in Star Trek: The Next Generation. Mark Lenard's memorable appearance as Spock's father revealed Sarek to be afflicted with a rare disease that stripped away his emotional control, a terrible state for a Vulcan. But the effects of this disease led him to confess, for perhaps the only time in his life, his love for his son.