The Three Companions of the title are Polly and the Brigadier, both repeated from their TV appearances by Anneke Wills and Nicholas Courtney, and Thomas Brewster, who was written into several of last year's Fifth Doctor audios, played by soap star John Pickard (and who at one point went off by himself in the Tardis, which is presumably the point where this story is set). It falls into three separate cycles: in the first four episodes, Polly tells the story of visiting a dying planet with the Doctor, Jamie and Ben; in the middle four, the Brigadier tells her his story of a trip with the Third Doctor to a peculiar place with similar problems; and at the end, the three join together to battle global warmng in today's London (Jo's husband Cliff Jones makes an off-screen appearance).
The first episode, which tells the story of Polly and the Brigadier meeting, is entirely brilliant and lovely, and the next three telling her flashback story keep up the momentum. I'm afraid I thought it deteriorated as it went on. I wasn't a fan of Pickard's performance as Brewster in the first place, and he hasn't improved much. The actual conspiracy didn't seem to make a lot of sense, and nor did the denouement.
Polly talks of her office in Westminster and says at the end that she "has a ministry to run", and I see some frantic fan speculation that she is meant to be a member of the British government. I think she is being flippant; she presumably holds a senior administrative job, that being where her career was headed when we first met her in 1966, but she is not a politician - if she were, the Brigadier would have heard of her (and she would have heard of him rather earlier).