A church glee club slaughtering a biker gang signals the start of an angelic civil war. The angels led by Bartholomew are being challenged by a faction led by an anarchist angel called Malachi (Stephen Monroe Taylor), with the idea that the winning side will challenge Metatron (Curtis Armstrong) to retake Heaven.
Human Cas shows up to help Sam and Dean with their investigation into the biker club massacre, but Ezekial - the angel currently possessing Sam - airs his continued annoyance about the presence of Cas to Dean, so Dean and Cas agree the former angel should depart again.
Unfortunately, Cas falls into the hands of Malach's men. However, the biggest revelations are going on 'behind-the-scenes' as Meatron meets up with Ezekial and reveals that he knows the angel inside Sam is not Ezekial, but Gadreel - who had been imprisoned in Heaven up until The Fall.
In the Supernatural version of mythology, Gadreel had been the guardian of the Garden Of Eden and was being punished for allowing the Serpent (Satan) to enter.
Metatron has his own plans about the future of Heaven, but requires Gadreel to prove his allegiance to the cause.
Holy Terror was a slow build, but with a powerful climax, ramping up the season's overarching storyline, killing off (unexpectedly) a key character, getting Cas back in the game and generally shaking things up.
About the only benefit of being a season and a half behind the States is that while this episode was the Season Nine mid-season finale at home, there won't be any waiting around for the next episode here as E4 is crashing on with the series next week, without any loss of momentum.