Jump-the-shark moments are typically scenes that finally convince viewers that the show has fundamentally and permanently strayed from its original premise. In these cases, they are viewed as a desperate and futile attempt to keep a series fresh in the face of a decline in ratings. In other cases, the departure or replacement of a main cast member or character or a significant change in setting changes a critical dynamic of the show.
The phrase specifically arises from a scene in the hit TV comedy series Happy Days. During the show’s run, the writers were challenged to come up with new, fresh stories; they developed a story where Arthur “Fonzie” Fonzarelli is on water skis, wearing his trademark leather jacket despite the well-known negative effects of salt water on leather, and literally jumps over a shark.
Part II of the episode: “Fonzie” reveals his ‘new‘ plan for Iraq.