Jumping the shark

"The term jumping the shark alludes to a specific scene in a 1977 episode of the TV series Happy Days when the popular character Arthur "Fonzie" Fonzarelli literally jumps over a shark while water skiing. The scene was so preposterous that many believed it to be an ill-conceived attempt at reviving the declining ratings of the flagging show. The phrase has become a colloquialism used by U.S. TV critics and fans to denote the point at which the characters or plot of a TV series veer into a ridiculous, out-of-the-ordinary storyline. Such a show is typically deemed to have passed its peak. Once a show has "jumped the shark" fans sense a noticeable decline in quality or feel the show has undergone too many changes to retain its original appeal."

Kilde:
http://en.wikipedia.org/wiki/Jumping_the_shark