Jump the shark
To reach the point in a TV series that denotes it is irretrievably past its best by introducing some ridiculous or otherwise unbelievable plot device or characterisation in order to boost ratings.
The phrase derives from a scene in the three-part 'Hollywood' episode of the American TV series Happy Days, broadcast in September 1977. The scene has 'The Fonz' (Henry Winkler), water skiing - unaccountably still wearing his trademark leather jacket - and jumping over a shark.
The episode, which came at a time when the long-running series was already considered past its best, was seen as the a virtual admission by the programme's makers that the series had run its course and that they were deficient of ideas. The phrase has been adopted in the USA - although it is rarely heard elsewhere - an has become a stock item in TV critic's reviews whenever a show loses its way.
Long-running US TV shows can be exceedingly lucrative and it is perhaps understandable that executives who profit from them don't want to kill off their milch cow. Examples of shows that have gone on too long are legion; in fact, examples of successful and long-running shows that haven't are few and far between. One such is The Simpsons cartoon series, which, by common consent and despite occasional lapses, has generally maintained a level of consistency throughout many series. Such a popular and long-running series is particularly susceptible to accusations of running out of ideas and 'jumping the shark'. The show's writers appear to be well aware of this and have made numerous knowing references to it, including parody scene of Homer Simpson water-skiing over a shark.
See other phrases that were coined in the USA.