non sequitur

Having a costumed superhero abduct the vicar was an utter non sequitur in the novel.

Ralph Wiggum is generally employed as a bottomless fount of glorious non sequiturs, but in “I Love Lisa” he stands in for every oblivious chump who ever deluded himself into thinking that with persistence, determination, and a pure heart he can win the girl of his dreams.

Coordinate term: paraprosdokian