I just listened to this audio book. I had to talk about this. I normally don't review ANYTHING, because well, I suck at these sort of things. I'm far too brief and I hate giving things away. But this will be an exception. Mostly because it's buzzing in my head with the awesomeness.
I have been listening to the Trilogy by Big Finish with The Doctor, Turlough, Tegan, and Nyssa. I loved the first two stories. They were well done, but this one, the third in the story is just wonderful. The Cradle of the Snake is, of course, the return of the Mara. It's now my favorite Mara story.
For a brief moment, there was sheep!Nyssa and sheep!Turlough, which was great. My sister loved it. Mark Strickson said he loved doing that, then joked that he was used talking to sheep.
If you thought evil!Tom was creepy, wait till you hear evil!Davison. It's creepy as hell. It's not often where the Doctor himself is the bad guy.
I really can't say anymore because it's against my nature, but I do recommend it to Doctor Who fans. Especially if you love Classic Who and the Fifth Doctor.