 Directed by Mark Kirkland , " Lisa the Vegetarian " is the first full @-@ length episode David S. Cohen wrote for The Simpsons . David Mirkin , the show runner at the time , supported the episode in part because he had just become a vegetarian himself . Former Beatle Paul McCartney and his then wife Linda McCartney guest star in the episode . Paul McCartney 's condition for appearing was that Lisa would remain a vegetarian for the rest of the series . The episode makes several references to McCartney 's musical career , and his song " Maybe I 'm Amazed " plays during the closing credits . 
