For two whole seasons, Sherlock could do no wrong. Over those first six episodes, Mark Gatiss and Steven Moffat created one of the best detective shows ever, not only striking gold by casting Benedict Cumberbatch and Martin Freeman as Holmes and Watson, but also having the narrative chops to create some genuinely compelling crime mysteries.
After wrapping up the main narrative at the end of the sixth episode though, which saw the series-long villain Moriarty commit suicide and Sherlock faking his own death, the creators had trouble finding a way to justify even more adventures for TV's best dynamic duo.
Consequently, while the next two seasons (and the Christmas special) still have their moments, the quality of the plots and the overall writing couldn't hold a candle to those earlier episodes, leaving the two leads to do most of the heavy lifting.
Where it used to promise a tight and concise crime thriller every single week, by the time the fourth season came around the narratives themselves played second-fiddle because of just how bloated the episodes had become, with the characters themselves being the only reason to justify tuning in every week.