A rare two-parter - first since The Rescue in 1965, and the last until Black Orchid in 1982 - and a nice, tight little continuation on from Ark in Space. There's not really a whole lot going on in this story, so it feels to me to be a little bit of a "fill-in." Nevertheless, its nice to see the Sontarans again and Styre's mission to experiment on humans is a little dark.
This was shot entirely on location and entirely on video. During production on location, Tom broke his collarbone, but with the scarf and duffle coat covering up his neck brace, and stunt double Terry Walsh shot from behind, its largely unnoticed.
Its a really good one to watch if you need a "quick fix" for 50-odd minutes. Like I said...a nice, tight adventure.