The idea of having Liz Shaw and Jo Grant together is good and well handled. Not only is the Doctor's universe under-populated as far as sexuality is concerned it is also a curiously sanitised entity. I mention this as prudery and stoicism in aristocratic circles in Russia when all those Winter nights were so cold does seem to take romanticism a bit too far. Never a truer word said. It's got some real problems, but somehow this works.
Characterisation is all over the map, but the action scenes hold it together.
Let's get the most obvious and serious problems out of the way first. The regulars are awful. They're just flat-out wrong. Not only in representation, but in their very setting for this book. Okay, I take that back.