This is a "companion" cd of sorts - instead of the more usual narrative of a story involving the Doctor told by one of his companions, this is the story of the UNIT vault - the place where artefacts, alien or otherwise, are kept safe and under control by UNIT. In the course of the story, we are introduced to a new UNIT member of the Vault team, who is in turn inducted into what the Vault keepers do - and so we hear about it as well.
It's a fairly slow moving audio experience; there was an awful lot of exposition done in the "story" through the old "Of course! ..." or the "You mean ..." trick where the painfully obvious point being portrayed in the story can (again) be spelled out for the listener by the character. I found this rather too thickly laid on and a bit annoying after a while.
The stories seem quite disjointed to start with, as we are told small tales of earlier Doctors and the links to the artefacts in the vault being studied; but, while they do come together in the overall story by the end, it all seems a bit laboured and, sadly, rather dull.
The highlight for me was hearing parts of the story narrated by the characters of Jo, Romana, Zoe and Stephen. Fairly good; but not great.