Bart,with Lisa's inadvertent assistance,accidentally knocks Martin Prince over a cliff and he is presumed dead. As Bart's guilt feeling grows Nelson turns Columbo-style investigator to prove that foul play was involved,although all is resolved amid Martin's butterfly collection when he 'returns' from the dead. Marge gets a documentary crew to shadow and film Homer as he is clearly cheating on his diet.
—don @ minifie-1