Castle has been a fan-favorite series since its 2009 debut. Following the murder-mystery adventures of Detective Kate Beckett and writer Richard Castle, the show gave audiences a perfect balance of lighthearted comedy and sincere drama throughout its tenure.

But occasionally, it would take a darker turn for certain stories, one of which almost served as a backdoor pilot for a new spinoff series.

In the Season 3 two-parter, “Setup/Countdown”, the investigation of a murdered cab driver takes a turn for the worse when it’s revealed to be part of a domestic terrorism plot to destroy New York City.

The story then introduces audiences to DHS Agent Mark Fallon, played by Adrian Pasdar, who takes over the investigation and begins tearing through suspects in search of a hidden nuclear weapon.

As a part of Castle, the story works well, though it may stand out a bit from most episodes of the series. But that may just be because it was intended as the start of Agent Fallon’s very own show.

Looking at the two-parter as a whole, it seems that a very clear and intentional focus was placed on Mark Fallon.

 The story took time to flesh him out as a three-dimensional character, establishing that he has a clear set of morals and even giving him a tragic backstory; his wife was killed in the 9/11 attacks, which drives him to prevent something like that from happening ever again.

It’s a lot of focus on a seemingly one-off character, who never shows up in the series afterwards.