I do really like stories like this that are just an absolute nightmare scenario for the Doctor and their companion(s). I'm a big proponent of the philosophy that beating the hell out of your characters is how you get some of the best character moments and this serial does not disappoint.

Five sacrificing himself to save Peri at the end was incredible and heartbreaking, and you have to feel a bit horrible for Peri because then she has to put up with Six for the rest of her tenure as a companion, and that probably weighs very heavily on her.