Danger Down Below was the initial short story in the 1983 Doctor Who Annual. It was an entirely off-Earth story featuring mostly the Fifth Doctor and Tegan, with a cameo by Nyssa. It introduced Threll, a character who had known the Doctor in a different, but unspecified, incarnation and who was portrayed as being one of the Doctor's oldest friends.
The Doctor has landed on the planet Aronassus 49 in response to a general distress call from his old friend, High Minister Threll. He's not quite sure why Threll is in trouble, but their friendship makes answering the call a priority. After a long trek through the desert surrounding Prime City, the Doctor and Tegan finally are escorted roughly to Threll by the city's guards.
Threll is considerably happier to see the Doctor than his guards were. He marvels for a moment at the Doctor's regenerative powers before explaining the problem. The city's food synthesisers are on the fritz, and the city's population has now begun to starve. Catastrophe is imminent, unless they can be repaired. Unfortunately, Threll has no idea what exactly is wrong with them, because he doesn't have clear access to them.
The Doctor, naturally, wants to examine the problem up close. As the trio descend to the city's service areas, their progress is temporarily thwarted by Chief Guard Sholl, who wants to arrest Threll. Sholl alleges that Thrall is derelict in his duties, and has in some way exacerbated the conditions of the famine. He goes on to say that the penalty for these crimes is immediate death. Just at that moment, an electrically charged ball bounces into the area, touching all of the security guards. They are all instantly incapacitated.
The Doctor surmises that the ball was a projection of a larger alien body down in the foot synthesiser area — a kind of defensive mechanism, much like the human body's white corpuscles, but more sentient. It realised that the guards were threatening Threll and the Doctor, whom it somehow sensed would be more sympathetic to its host.
The trio continue to make progress towards the synthesisers. Once they arrive, they immediately see the problem. A giant creature has taken over the room and is consuming the food that the machines produce. The Doctor identifies it as a Mirator from the Andromeda Galaxy, and realised that he must somehow get the Migrator back into space. Its long journey from the Andromeda Galaxy to an uninhabited world was interrupted by the binary star system around this planet. The two hot suns awakened the creature prematurely from its state of suspended animation and forced it to land. If he can get it back into space, things will return to normal on Aronassus 49.
The Doctor rushes off to get the TARDIS, leaving Tegan and Threll to deal with the guards who have now awakened from the slumber caused by the Migrator's symbiotic "friends". The energy balls leap back into action to help stave off the guards. In the midst of this fight, the TARDIS materialises and whisks the Migrator, Tegan and Threll away. The guards are left to find and begin to distribute whatever food the Migrator hasn't yet eaten.
In space, the Doctor, Nyssa, Tegan and Threll manage to get the Migrator back in suspended animation so that it can complete its journey to an uninhabited world. The Doctor then proposes they have a big celebration. Threll is shocked; surely it would be so unseemly so close to the near-starvation of his people.
The Doctor agrees, but adds, "That's why being a Time Lord comes in so handy sometimes." He can use the TARDIS to get Threll back home seven days in the future, by which time his people will regard Threll as a hero and a party will be in order.
- The Doctor is said to have a dual respiratory system.
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