3.4 Mad Dog. It reminds me obscurely of the very late Avengers episode Take-over, where Steed, like Charles, is thrown on his own resources and chased from pillar to post. And I love the moment where the [spoiler] gradually appears through the steam to save Charles. It's just about the last thing we'd expect to see.
It also has Captain Dent shouting 'I'VE GOT RABIES'. While I was waiting for my COVID test to come back I was practicing that line, and also Dr Tyler in 1.1 saying 'I've got the disease', and internally debating which one to use online if the test was positive.
Which it wasn't.