I enjoyed this quite a bit. It was an interesting look into Peri and Erimem's psyches. I only wish that they could have come up with something a little more original than the old 'an alien life form alters Our Heroes' behavior in some way' trope.
I continue to enjoy Peri as a character. Big Finish shows us that she /can/ be an interesting, relatable character, despite her less than stellar on screen appearances. It just goes to prove that it's all in the writing.
I'm surprised that they are telegraphing Erimem's exit so soon--in fact, they've been doing so it little ways throughout her series. I'll be sorry when I hear her last audio--the character really did add something to the dynamic in the TARDIS. Five, Peri and Erimem have become favorites of mine, quite by my own surprise.