This was the first Creator/BigFinish episode to feature the shape-shifting companion Frobisher, from the ''Magazine/DoctorWhoMagazine'' comics. It's also Creator/StevenMoffat's favourite Sixth Doctor episode in all of ''Series/DoctorWho'', and it was a clear influence on his own [[Recap/DoctorWhoS35E11HeavenSent "Heaven Sent"]].
[[EverythingsBetterWithPenguins Frobisher]], the shapeshifting penguin detective, has convinced the TARDIS to create semi-living fish for him to toy with. The Sixth Doctor tries to explain that even semi-living things can feel pain and distress, but Frobisher won't listen until the TARDIS herself can't cope with Frobisher's games anymore and shuts down entirely. She parks the Doctor and Frobisher inside a medieval-style castle, where the new god-king is about to be crowned.

The WorldOfSnark is inhabited by people whose entire lives revolve around rituals. The old king had ritualistically married his pretty wife, ritualistically conceived a do-good son and grown to ritualistically deeply hate him. The queen had ritualistically slept with the captain of the guard, who was then ritualistically executed for fathering the ritualistic bastard son. This son, the new king's ritualistically evil half-brother, plans to depose his brother now that the old king has died. An old scribe notes down every moment of the king's life, gathering each new Bible in the library. Well, sort of a library. Well, his bedroom.

Upon arriving in the middle of the ceremony, the Doctor and Frobisher are instantly [[MistakenForGods mistaken for]] the Holy Big Talking Monochromatic Bird and his companion who brings with him every colour of the rainbow. The new king, who's really very confused about the whole deal, becomes even more confused when the coronation fails to give him actual god-like powers. He's too nice of a chap to pretend he's divine, so he promptly abdicates and tells the people to assassinate him for blasphemy. They won't listen, though, and he's forced to accept his new role. The new king's wife begins with the ritualistic torturing and mocking of the old queen (who's not in the mood), while the high priest goes off the ritualistically conspire against the new king together with the evil half-brother.

While Frobisher stays with the king (after observing the ritualistic assassination attempt), the Doctor soon finds out that the old scribe's books are all written in the same handwriting. Even the ones dating back to previous dynasties. What's more, each king's life ends exactly on the final line of the final page of each book.

The king decides that he really, really, ''really'' doesn't want to do any of this, so he makes Frobisher the new god-king instead. This does not go over well with Frobisher.

Meanwhile, the evil half-brother has a secret that's decidedly ''not'' tradition. He has a child, a young boy who's been kept alive for five years in the castle's deepest dungeons. The child was raised "pure", without language -- even his mother's tongue had been cut out during his birth. The child turns out to be a malicious God, intent on killing everyone it encounters. The Doctor is an unexpected factor in its plans, but -- as the child finds out -- isn't immune to MindRape. After violating the Doctor's memories, it goes off into the castle and starts murdering everything it can find... asking [[MadnessMantra "are you my father?"]].

In the end, it turns out that the evil half-brother wasn't the child's father after all. It was the old scribe, whose punishment this prison pocket-world revolves around. The old man had once killed his own son, and had created the fantasy of kings hating their sons and princes becoming god-kings as a coping mechanism. He had already lived the story many times over, in a continuous loop, always ending with the child's murderous rampage. When the TARDIS' dimensional stabiliser became damaged by Frobisher forcing her to create semi-living lifeforms, she had fled to the nearest dimensionally transcendental space going through the same kind of thing, in order to repair herself with its data. The scribe confronts his child for the final time, and ends the GroundhogDayLoop by commiting HeroicSuicide.

The Doctor tries to pretend that none of it mattered, since none of it was real, but Frobisher reminds him that even sort-of-living things count.

->'''The Doctor:''' If it's any consolation, they were never real in the first place.\\
'''Frobisher:''' No, Doc -- that's no consolation. They thought they were real, didn't they?\\
'''The Doctor:''' Just like the fish in the swimming pool... they felt pain, they felt fear... and more than that, they had hopes, and dreams and families. Yes. It is terrible. But that's what comes of travelling in the TARDIS. All the people you meet, all the planets you see: you know they won't last forever. And our next journey could be to a time when they'll all have been long forgotten. Such little lives... that we could feel like gods, set apart from them all.\\
'''Frobisher:''' And... that's supposed to make me feel better?\\
'''The Doctor:''' No. No, not at all.\\
'''Frobisher:''' For a while back there, Doc... I actually felt I could do some good! I actually felt I could save them all.\\
'''The Doctor:''' I know, Frobisher. Believe me... ''I know''.
!!The Holy Terror provides examples of:
* {{Absurdism}}: Capital-A Absurd black comedy is par for the course with Robert Shearman episodes.
* AbdicateTheThrone: Pepin's plan, but the "story" is trying to force him not to.
* AdiposeRex: The former empress, Berengaria.
%%* AndThenWhat
* BastardBastard: Childeric, although he's not evil enough in Berengaria's eyes. She's very, ''very'' wrong.
* BigBad: Eugene is this for the story, as he is the one who created the Castle as his SelfInflictedHell, killed his son in the first place, inflicting the emotional damage which will turn the Child into the killing machine he is, and though he is given the chance to pull a HeelFaceTurn at the end, he kills himself by way of the Child instead.
* BigBadWannabe:
** Clovis, as described by the Doctor in the fourth part.
** Childeric too, in a way, who is killed by [[HoistByHisOwnPetard his own creation]], the Child.
* BlatantLies: The [[Characters/DoctorWhoClassicSeriesDoctors Sixth Doctor]]: "The TARDIS, like her master, has her ego under control."
* BuffySpeak: Frobisher is nearly constantly referred to as "the big talking bird", which makes the line "All hail the big talking bird!" quite funny.
* BurnTheWitch: When a new GodEmperor is crowned, all those who worshipped the old one are burned at the stake. Although they are given a chance to recant, which most of them take.
* CallBack: To [[Recap/DoctorWhoS7E4Inferno "then you won't feel the bullets when we shoot you"]].
%%* ClarkesThirdLaw
* {{Cliffhanger}}: Three, as usual, but most notably Part Three's cliffhanger is especially shocking: [[spoiler:The Child's voice morphs into that of Eugene's as he asks who his father is.]]
* ContinuityNod: The Sixth Doctor is still [[Recap/DoctorWhoS22E4TheTwoDoctors opposed to fishing]].
* CorruptChurch: And how! The High Priesthood has a long history of betraying the true monarch to the evil brother going back to time immemorial.
* CreepyBasement: With a CreepyChild in it.
* DeathSeeker: Berengaria, Pepin and Childeric's mother, is this. [[spoiler:She is eventually killed [[KickTheDog moments after finally seeing her son in a new light, and just after said son was murdered in front of her eyes]] by the Child.]]
* DesignatedVillain: InUniverse even: The High Priest and the bastard brother. Not only Childeric but everybody before them in the GenerationXerox.
* DirtyCoward: Clovis despite the fact he was trying to gear up for a HeroicSacrifice in contrast with his nature.
* DoomMagnet: The TARDIS' tendency to do this gets lampshaded.
* DownerEnding: ''Big time''. It's a KillEmAll DreamApocalypse, which only the Doctor and Frobisher, since they do not belong in the dreamworld, survives.
* {{Egopolis}}: As soon as a new emperor is crowned, his people convert all the old statues into images of him.
* EnfanteTerrible[=/=]RealityWarper: Childeric is trying to groom his son into one. Well, ''Eugene''[='=]s son. And it works ''far'' [[GoneHorriblyRight too well]].
* EpiphanicPrison: Is this for Eugene.
* EverythingsBetterWithPenguins: Frobisher, the big talking bird.
* [[EvilChancellor Evil High Priest]]: As is designated by tradition, the High Priest must always betray the Lord Emporer.
* EvilPlan: The bastard son is obligated to have one for the rebellion, but Childeric's plan is ''much'' more evil than the Castle has ever seen before.
* EyeScream: If you don't pledge your allegiance to the latest God Emperor ''one'' eye is gouged out so you can watch yourself be burned at the stake.
* FauxAffablyEvil: Childeric
* FisherKing: Eugene has this effect on the reality.
* GenerationXerox: Invoked. Every generation has the abusive royal parents, a milquetoast heir, a BastardBastard who conspires with the High Priest to overthrow the heir, etc. It's the key to the truth about the place.
* AGodAmI: Played with, Frobisher is worshiped as being some sort of an angelic being ("All hail Frobisher! All hail the big talking bird!"). He is not comfortable with it at all. Also, the King is traditionally worshiped as a living God. This leads to problems, since each King invariably commits the ultimate blasphemy by ''dying.''
* GodEmperor: Though they're not really immortal.
* TheGrotesque: Childeric, Pepin's evil brother who plots to overthrow him. He appears to be the villain of this story, but he really isn't.
* GroundhogDayLoop: Unbeknownst to Eugene until the WeirdnessCensor shatters.
* HeroicSacrifice: The Captain of the guard for the people, although it's more of a StupidSacrifice, since the assassination attempt is always carried out with blanks instead of bullets, [[spoiler:at least until they take the Doctor's advice]].
** SenselessSacrifice: Pepin does this for his mother, and is killed by the Child. Depressingly, Berengaria is killed anyway.
* HoistByHisOwnPetard: Childeric is killed by the Child when he is unable to keep him under control.
* InsaneTrollLogic: As an emperor, Pepin is supposedly invincible so [[spoiler:the ritual assassination attempt is done with blanks]] because it wouldn't work anyway.
* {{Irony}}: Scribes are not important? [[SelfInflictedHell They most certainly are]].
* JourneyToTheCenterOfTheMind: Beginning [[spoiler:the moment The Doctor and Frobisher step out of the TARDIS.]]
* KillEmAll: Livilla, Arnulf, Childeric, Pepin and Berengaria are all killed off over the course of the story, in that order. The trope is then taken UpToEleven in the final part, where its revealed that the entire castle is ficticious, created by Eugene's imagination, and even he is killed by The Child in the end, leaving only The Doctor, Frobisher and the empty void.
* LadyMacbeth: Livilla, Pepin's wife.
* LastWords: The Child restores Arnulf's voice only to hear his before he kills him.
* LukeIAmYourFather: Subverted, and then played straight: [[spoiler:Childeric isn't the Child's father, but Eugene is.]]
* MadwomanInTheAttic: Childeric's son in the vault
* MindProbe
* MindRape: What's actually going on. The entire castle serves as Eugene's punishment for the [[MoralEventHorizon murder of his own infant child]]. The Child also does this to The Doctor at one point.
* MeaningfulName[=/=]PunnyName: ''Child''eric.
* MerlinSickness: Forced on Levilla.
* MoodWhiplash: One of the most well-loved examples in Creator/BigFinish. The serial switches from a hilarious parody of stereotypical Shakespearean tragedy and suddenly becomes a disturbing examination of destiny as the characters stare inevitable death in the face and a man's fractured mind is slowly torn apart before the Doctor and Frobisher's eyes.
** It's made even worse, as after the melancholy ending, we hear the ending theme followed immediately by chantings of "All hail Frobisher, all hail the big talking bird!" to a techno beat.
* MundaneMadeAwesome: Each God-King has to perform a miracle during their coronation. By the time the Doctor arrives, it's been pared down to a (rigged) card trick.
-->'''High Priest:''' Now, your Majesty. Is it by any chance... '''''[[LargeHam THE THREE OF CLUBS?!!]]'''''
* MyGodWhatHaveIDone: At the end of the story, Eugene experiences this when he remembers that he killed his son.
* {{Narrator}}: The scribe is this in-universe for the bibles.
* OffingTheOffspring: Eugene's crime and the reason why he has been placed in the castle.
* PocketDimension: The Castle itself is one.
* PrecisionFStrike: [[ThisIsForEmphasisBitch Alright, you evil old bitch!]].
* PsychologicalTormentZone[=/=]SelfInflictedHell: The Castle is this for Eugene, who is stuck in it forever because of his own deeds, namely murdering his son.
* RealityWarper
* SecretUndergroundPassage
* SharingABody: Childeric plans this. He dies.
* ShoutOut: When Pepin addresses his people, it seems very much like ''Film/MontyPythonsLifeOfBrian''.
* ThemeNaming: Characters are named after French or Roman royals famous for scheming and backstabbing. Tacitus is named after the general and historian of the same era as the Roman namesakes.
* TheThermidor: God-King, bastard brother, corrupt High Priest. Lather, rinse, repeat.
* ThereAreNoCoincidences: All of the bibles end ''exactly'' on the last page. Strangely enough, Pepin's seems to be very thin.
* TongueTrauma: Childeric does this in order to mute his servant, and almost to Eugene.
* TooKinkyToTorture: The former empress says back in [[NostalgiaFilter her day]] they REALLY knew how to torture.
* TheUnfavourite: Pepin is this to his mother, not that Childeric is any better in her eyes.
%%* VerbalJudo
%%* WellDoneSonGuy
* WhamLine:
-->'''The Child[=/=]Eugene:''' Are you my father?
* WorldLimitedToThePlot: Because the "plot" really is everything to this world.
* YourHeadAsplode: The {{Enfante Terrible}}'s rampage during the climax, complete with an incredibly disturbing ''tearing sound.''