In 1935, the Eighth Doctor and Charley have just finished defeating an insane {{sculptor|s}} and his living artworks, when they get a message on the TARDIS internal communicator. It's the Eleventh Doctor, of course, who says the Doctor has to stop the enemy aliens and that William Tell is the key. While Charley thinks this is a reference to ''The William Tell Overture'', the Doctor discovers a music hall memory man called William Tell, who leads him to a German spy ring in Scotland.

And then it turns out the message was about ''The William Tell Overture'' after all, and the Doctor and Charley have been following the wrong plot. Now they have to stop the Germans ''and'' the aliens.

!!'''Tropes'''
* AllMenArePerverts: When confronted by a walking encyclopaedia act, most of the crowd start out with base things such as Mae West's measurements.
* BavarianFireDrill: Hammond pulls one when the crowd of a theatre goes into panic after an assassination.
* BuffySpeak: Charley has a special dictionary for the Doctor's gadgets, ie, wotsits, thingymidooda, and once ''Oojamaflip''.
* CallBack: The title refers to both space aliens and agents of foreign powers; the same confusion that was central to [[Recap/BigFinishDoctorWhoDOTD1HuntersOfEarth the first release in the series]].
** Charley describes the TARDIS console as [[Recap/DoctorWhoTVMTheTVMovie mahogany]].
** Charley mentions to Hillary that she dressed up as [[Recap/BigFinishDoctorWho016StormWarning a boy before]].
* DoubleAgent: Hillary offers to be one when the British capture him.
* FaintInShock: Several members of the BBC faint when the Doctor, Charley and [[BreadEggsMilkSquick an eight foot tall glowing green alien]] enter the premises.
* FakeOutMakeOut: Hillary offers this when several army men get on the train looking for Charley while she's suspected as a murderer. Charley has to remind him that she's dressed as a ''male'' sailor, which, for the time period, wouldn't have been any better than him being caught assisting a wanted murderer.
* GoToAlias: A taxi carriage under the name "John Smith" gets arranged.
* {{Homage}}: To ''Film/{{The 39 Steps|1935}}'' - Charley visits a memory act at the music hall, is framed for a murder she didn't commit, then flees Portland Place to Scotland in the company of a stouthearted Englishman very like Richard Hannay to investigate a spy ring. [[spoiler: And then things start to go (metaphorically) off the rails.]]
* IKnewIt: The Doctor in regards to the prerecorded message, he knows the contents beforehand.
* JustInTime: The Doctor and Charley stop the William Tell overture as it starts. Then subverted when it turns out those few notes were enough.
* KilledMidSentence: Mr. Tell.
* LostInTransmission: For the first time in ''Destiny of the Doctor'', the Doctor only gets half Eleventh's message. It's implied that this is why the other messages have been left in more obscure places.
* MadArtist: In the adventure the Doctor and Charley have just completed at the start of the story.
* ManchurianAgent: Triggered by the William Tell overture.
* NeverMessWithGranny: The two elderly Scottish ladies Charley and Hillary meet on the train. [[spoiler: Then it turns out they're part of his spy ring. And then it turns out that they're not, they're Special Branch infiltrators who call in TheCavalry!]]
* NoodleIncident: The battle against the mad artist. Involving a ''lot'' of NoodleImplements.
* {{Pun}}: When the Doctor unleashes a sonic barrage on the electronic fog.
--> '''Charley''': Doctor that hurts!
--> '''The Doctor''': Not hertz! TERRAHERTZ!
* RetCon: The reason why the 11th Doctor has been leaving messages in esoteric places for his previous incarnations is because of the fog in the current story.
* SelfDeprecation: The Doctor describes as the only person that can leave a message on the internal TARDIS message circuit is a smug, superior bossy booted idiot, with a pretence of being an enigma hoping to trick the Doctor into helping him. Ergo, himself.
* SmugSnake Hillary has traits of this.
* StandardSnippet: The William Tell overture.
* SweetPollyOliver: Charley dresses as a young boy.
* TheThirties
* TitleDrop: "And watch out for those Enemy Aliens!"
* TriggerPhrase: The William Tell Overture.
* UnresolvedSexualTension: This is the phase where Charley still harbours feelings for the Doctor, whilst the Doctor is his naive CuddleBug self.
* WronglyAccused: Charley picks up a [[IdiotBall smoking gun after an assassination]], which leads to this.