For instance, Ace's sacrifice of her hearing was a lovely, inspired touch. Sophie Aldred plays that scene wonderfully, and while some of the jokes were predictable it would have been sad if they hadn't been made. She is violent when she needs to be, but it isn't her first choice of methods. (She does, however, still think confronting the enemy is the way to go. The minute she yells "Oi!" I start grinning.) Ace is back, and back to having faith. It's not the unreasoning faith of her youth, but the tested faith of someone who has made the choice to believe.
Joining her in that faith we find Bennie, and she is fantastic. I particularly loved her comments about Eight, who doesn't play an active role but whose existence provides a plot point. Her partnership with Ace is beautiful and forms an important fulcrum on which the Doctor can move events. They have come to an understanding. Bennie is equally capable of snark, but she has an innate tenderness. Her intelligence gets them through multiple bad moments, and the aforementioned faith is pivotal in fixing the situation.
The Doctor is less a factor than normal as far as actions go. His primary role is to struggle with himself and the Scourge. While this lets the humans play the most active part in events, it means we get a lot of what comes dangerously close to whining. On the plus side, McCoy gets some lovely lines and he does a great job when he's allowed to be reflective. The end, too, is well-acted.
Michael and Annie's story comes to a more-or-less satisfying end. So does the story as a whole.
I would not advise listening to this story with a headache, by the way. The sound effects can get a bit harsh.