There's a secret locked up in UNIT's Vault 75-73/Whitehall. Dr Elizabeth Shaw is the only one left who knows what that secret is. Returning to UNIT for the first time in decades, she slowly unravels the past. The vault contains the remains of a spaceship that crashed in the Pennines in the seventies. For the young Liz Shaw, the priority is to ensure the thing's safe. However, the Doctor is more concerned about the alien pilot and the chance this ship offers for escape. Can he resist the temptation, or will the Third Doctor turn on his friends?
During one of his experiments, the Third Doctor was alerted to the presence of an alien spaceship falling to Earth, so he went up to meet it on its arrival in the north of England. At the landing site, he was eager to open up the capsule but was persuaded out of it and to do it under controlled conditions. Back at the lab, he opened the capsule and discovered the remains of its pilot, a Mim. When the Doctor touched the Mim, he writhed in pain and ran to the TARDIS. Liz later informed him that he was impersonated by one of them so the Mim could disable the Earth's defences and pave the way for their invasion. When Liz came into the TARDIS, she found him collapsed and informed him of what had happened whilst he wasn't there, and in turn told her why he went to the TARDIS. The Doctor asked for the Time Lords' help to deal with the Mim as the destruction of the humans would damage the Web of Time. The Time Lord messenger told him that they couldn't interfere with what was happening on the surface of the planet but informed him that they would deal with the Mim invasion flotilla. The Doctor used this to his advantage and managed to lure the Mim scout from their battle with UNIT back to his ship where he planned to detonate it and kill the Mim. Unfortunately, Sergeant Robin Marshall was caught in the explosion, which eventually led to the promotion of Corporal John Benton to Sergeant.
- Liz comments wryly upon the oddly British practice employed by UNIT of "hot sweet tea" salving all wounds.
- This audio drama was recorded on 11 August 2009 at The Moat Studios.
- Liz refers to Mike Yates as "Captain Yates", which is at odds with his rank as given in the novel The Scales of Injustice, but is consistent with the audio story Vengeance of the Stones.
- The Sergeant who dies in the story is stated to be Benton's immediate predecessor, thus setting this story before TV: The Ambassadors of Death.
- This story was told from Liz's perspective.
- This story was originally set on CD and download.
- The Doctor and the Brigadier are still not back on good terms after the bombing of the Silurian base on Wenley Moor, a few weeks earlier. (TV: Doctor Who and the Silurians)
- Liz comments that her dissatisfaction with her role in UNIT arose because she felt all she had to do was tell the Doctor he was brilliant and pass him his test tubes. She later told the Brigadier this face-to-face, (PROSE: The Scales of Injustice) which he later used to convince the Doctor to accept Jo Grant as his new assistant. (TV: Terror of the Autons)
- The Doctor is said to still be deeply offended at being imprisoned on Earth. (TV: Spearhead from Space)
- The Time Lord in a bowler hat appears in the TARDIS. He would later appear to warn the Doctor of the Master's arrival on Earth. (TV: Terror of the Autons)