"Nuking the fridge" refers to the moment in a film series when it becomes apparent that a certain installment is not as good as previous installments, due to ridiculous or low quality storylines, events, or characters.
Originating from the film "Indiana Jones and the Kingdom of the Crystal Skull," Harrison Ford's character survives a nuclear detonation by getting into a lead-lined fridge before being blown thousands of feet only to crash back down and emerge safely.
The absurdity in this occurrence is that, although lead is radiation proof, lead melts at 620°F and is not resistant to the millions of degrees, that say, an atomic bomb would put out. This is the prime example of the lower quality and outlandishness disgorged as a continuation of the series, and thus the phrase was coined.
The saying is also a reference to the phrase "jump the shark
," referring to an episode of Happy Days where the Fonz
jumped a shark on water skis, considered the lowest point of the show. "Jumping the shark" is applied to a television series alternatively to film.