With Hex's memories lost and their old friend now reduced to the amnesic Hector Thomas, the Doctor and Ace's first trip with Hector leads to the Doctor facing the return of the Swarm ("The Invisible Enemy"), as it simultaneously ensures its own creation and sows the seeds for its return.
This audioplay provides examples of the following tropes:
- Chekhov's Gun: The Doctor retains his old immunity to the Swarm as he still has the antibodies in his blood that he acquired from Leela during his original encounter with it.
- Expendable Clone: The Swarm's plan includes creating a clone of itself so that it can upload a digital copy of its mind, which survives the death of the clone's physical form.
- It's All About Me: The Swarm consistently refers to its creation as the single most important moment in the history of the universe.
- Mega-Microbes: The Swarm uses the TARDIS's relative dimension stabiliser to make itself larger, with the goal of eventually making itself so big it would be the only living thing in the universe.
- Nice Job Breaking It, Hero: The scientist who created the Swarm only intended to augment an existing plague so that she could use it to help immunise people from other diseases, and it's possible that the Swarm manipulated her work anyway.
- Puppeteer Parasite: The Swarm, as before, although it is initially only able to manifest in Hector due to the 'gap' in his mind.
- Sequel Episode: The whole storyline is basically one for "The Invisible Enemy", albeit one that also sets up the creation of the Swarm.
- Small Name, Big Ego: The Swarm is literally microscopic but considers itself the only life-form with any right to exist.
- Timey-Wimey Ball: The Swarm basically ensures its own creation a few centuries after its 'death' (based on the assumption that the Swarm has been in the TARDIS since the Fourth Doctor's era) by going back to the moment of its creation, and later goes forward in time a few centuries to ensure its return.
- The Virus: As always, the Swarm is this.
- What the Hell, Hero?: Hector gives one to the Doctor and Ace at the end as he is put off by their cavalier attitude to how many people have died during the crisis.
