Best Example: Buffy the Vampire Slayer
The infamously bleak sixth season of Buffy isn't bad per se, but it was just a far cry from the series viewers were used to for the five preceding years. While not without drama, Buffy was always notable for its quippish heroine, peppy dialog, and warm ensemble cast amidst the demons and destruction. The sixth season, in which the slayer readjusts to life after death, finds Buffy and the rest of the characters in wayward transitional phases and thus, appropriately gloomy moods.
It was a necessary step, but the seventh season was less a breath of fresh air than a sigh of relief that the series was back to its core essentials, with each character having worked through their darkness in the interim. The theme of returning to the basics is symbolized by the return of Sunnydale High, the series' main setting in its early years, and a villain that literally paid homage to the deep rogues gallery of seasons past.