S03E21 Black Widower
Marge's sister Selma has been writing to Sideshow Bob whilst he was in prison, and, on his release announces their intention to marry. Bart is appalled. Unlike everybody else he is unconvinced that Bob has changed and believes that, as Selma has come into money, he will attempt to murder her on their honeymoon. And he is right. Putting two and two together Bart works out how Bob plans to blow Selma up and manages to thwart him in time.