There's not a lot to this plot, but what's there is done marvelously. It's a great companion introduction for Vicki, with some compelling character conflict over Barbara's well-intentioned but disastrous killing of Sandy. It was also nice to see the Doctor feeling morose after Susan's departure -- it gives even more emotional weight to the end of the previous story. The evil plot, though, was very Scooby-Doo, and I don't just mean the costume; Bennett was literally the only other person on the planet. It isn't exactly Agatha Christie. It's a fun one to watch, though!