'Jumping the shark' is right, and its usage has become more generalised to include other media and even real life events. The phrase comes from an infamous 'Happy Days' episode in which the Fonz jumps over a shark on water skis.Sounds like "jumping the shark", but that applies to a TV series rather than a story.
….or perhaps:I like the phrase "Bollocks". Succint and too the point.
Horsefeathers is good. Courtesy the Marx Brothers.….or perhaps:
That's the point at which some part of my brain mutters to itself "Here we go..."Generally: the tipping point.
Also a fantastic Tears For Fears song.