If you asked me to explain why I think single-disc stories are the way forward for Big Finish, I'd point to The Cradle of the Snake. Not because there's anything horribly wrong with it, but because its flaws would be much less noticeable at shorter length. The return of the Mara written by Marc Platt ought to be an instant classic, but it uses its ideas so sparsely that all the thematic layering and character potential drain away, leaving a standard Doctor Who runaround.
The Mara is still in Tegan's head. The Doctor is worried, and so are Turlough and the much older Nyssa who, at least in this story, never acts a whit different than she always did. The Doctor takes steps to remove the Mara, and if you are the world's biggest chump you will believe for a few minutes that he succeeded. If you are not the world's biggest chump, you will immediately know what the Part One cliffhanger will be, and what the first bit of potential the story's going to waste will be.
Peter Davison is very good at playing the Mara-riddled Doctor throughout the story, but there's little sense of the kind of chaos a man of his intellect might be able to sow. His plan is an obvious use of the situation he finds, and while there's one small moment where he plays a couple his companions against each other, it is only one moment, and it comes to nothing in how the plot unfolds. Eventually the Mara gets ahold of Nyssa, and Sarah Sutton is great at giving her a patronizing hauteur, but in the long run it has nothing to do with anything. The Mara is only in her so she'll have something to be doing while Tegan and Turlough save the day.
Save the day, I should note, by doing something very like what was done in a previous Mara story, but without any psychological subtext: it's just a literal [SPOILER]. There's an attempt at mystical atmosphere via a one-note spiritual wise man, but his wisdom isn't moored to anything specific that the story might be about. This script knows the words of a Mara story, but not the music. There's also what the cover copy calls an "infotainment impresario," who I guess is a satire of celebrity presenters or telethons, or something, but this amounts to a couple good jokes that aren't really taken anywhere.
I can't help contrasting this with Fallen Angels, the fifth Doctor story from the first Classic Doctors, New Monsters box, which I also listened to recently (and will eventually review, I hope). That story doesn't really work either, also because it doesn't know how to build on its source material, but it's much more entertaining, because it's allowed to get in and out in 55 minutes rather than hanging around for almost twice that long. It doesn't spend its middle 50% treading water, which makes the thinness of its concepts less obvious, and the jokes hang together rather than hanging separately. The Cradle of the Snake is never noticeably padded, but all it actually does is combine different characters in various ways, repeating pieces of information as they move from one group to another. It feels like an exercise in filling out the season, which is just not what the Mara deserves, to say nothing of the audience.