This audio had quite a tough job to do - it had to create an alternate 3rd Doctor and then posit a new set of adventures for him based on the premise that he began his exile on earth in 1997 rather than 1970-ish, thus arriving in a world that had had to cope with any number of alien threats without his assistance. Jon Pertwee's Doctor was so iconic, and his stories so much of their time, that I really wasn't sure that this alternate imagining would be anything more than a cute thought experiment. It ended up being far more effective than I'd anticipated.
David Warner makes an utterly convincing Doctor. He's not playing Pertwee here, but he has some of the prickliness and the haughtiness of Pertwee's Doctor, and he very effectively conveys the melancholy of being an exile. He seems like a less flamboyant Doctor - I don't picture him in velvet smoking jackets. (I can't remember if the audio says anything at all about his mode of dress.)
Nicholas Courtney, as the only returning actor from the TV series, has a very important role in selling us on this new Doctor, and he does it very well. It's a bit hard, at first, to see this bitter, apparently washed-up version of the Brig, but it's wonderful to see the character come to trust the Doctor and, by the end of the story, have a chance at all the adventures he missed out on.
Mark Gatiss is also great as the Master.
The plot pays homage to the motifs of the Pertwee era (even including a Buddhist temple), while feeling contemporary. David Tennant turns in a great performance as a UNIT colonel with an antagonistic relationship to the Brigadier. There's some snappy dialogue.
If Big Finish were to make a whole season of audios with this lineup, I'd buy them in a heartbeat. With stories this good, who cares if they're canonical?