When a television series "jumps the shark" (see the Jump The Shark site) it "includes an over-the-top scene or plot twist that is indicative either of an irreversible decline in the show’s quality or of a desperate bid to stem the show’s declining ratings". The phrase comes from a season finale cliff-hanger episode of "Happy Days" in which Fonzie jumped a shark on water skis. Fonzie was on the skis, not the shark.
The show had arguably lost it before that point or Arthur wouldn’t have made the jump at all. Its decline began when Chuck Cunningham, the oldest Cunningham child, was first played by two actors and then vanished so thoroughly that by the time the show ended his mother wouldn’t even acknowledge that he’d ever existed.
I know what happened to Chuck.
"Happy Days" obviously takes place in some alternate universe; the 1950s and 60s didn’t look anything like the show. In the "Happy Days" universe the events chronicled in the 1950s classic movie "Invasion of the Body Snatchers" are a reality. Being pod people would explain the bizarre behavior of the cast.
Armed with this knowledge it becomes apparent that Chuck, a scholarship athlete and A student, thus a perfect 1950s teen movie hero, resisted assimilation by the pods. We’ll probably never know how long Chuck resisted before being destroyed or how many people he saved from the horrors of inhaustion, but it’s obvious that television would have been a much better place if he’d survived to triumph and save us from the last eight seasons of "Happy Days".
That should keep you for a couple of weeks.