Indiana Jones and the Kingdom of the Crystal Skull (2008)

Indiana Jones and the Kingdom of the Crystal Skull (2008) infamously derailed the adventure series with a 78% Rotten Tomatoes score, far worse than #2's critical reception. Steven Spielberg and George Lucas brought Harrison Ford back for aliens and a nuclear fridge survival scene that coined "nuking the fridge" as a pop-culture synonym for jumping the shark. Despite grossing $790 million—30% higher than the average franchise sequel—its cartoonish plot and Shia LaBeouf's weak performance mark it as the series' biggest letdown.