1899: The Doctor and Nyssa visit Stockbridge at Christmas, taking in the local flavor, including the performance of a traditional play which includes a doctor who is said to be “an earl of space and a lord of time.” Unnerved by the specificity of that reference, the Doctor sets out to discover the origins of the play.
1199: The Doctor and Nyssa discover strange goings-on in 12th century Stockbridge, from French knights and local noblemen who are not the people they claim to be, to a small Rutan task force intending to take over Earth to serve as a base of operations in Rutan war with the Sontarans. The Doctor will stop at nothing to keep the Rutans from achieving their aim. Nyssa, on the other hand, will cheerfully give them everything they want.
written by Alan Barnes
directed by Barnaby Edwards
music by Richard Fox & Lauren YasonCast: Peter Davison (The Doctor), Sarah Sutton (Nyssa), John Sessions (Roland of Brittany/Mummer), Joe Thomas (Hubert, Earl of Mummerset/Mummer), Richard Cotton (Osbert/Mummer/Yokel/Demon), Susan Brown (Maud the Withered/Yokel), Teddy Kempner (Yavuz/Mummer/Yokel/Demon), Trevor Cooper (Smithy/Mummer)
Notes: This is the first appearance of the Rutans in a Big Finish audio story. A single Rutan was the cause of the events of the Tom Baker Doctor Who story The Horror Of Fang Rock, which also established the Rutans’ ongoing war with the Sontarans. Another Rutan was seen – though not by the Doctor – in the fan-made video production Shakedown: Return Of The Sontarans (1994), the only time the Rutans and their mortal enemies have ever shared screen time. The Doctor says that the Rutan’s presence in England in 1199 is no coincidence: this Rutan crew was probably tracking the Sontaran soldier Linx (The Time Warrior, 1973-74), so it’s reasonable to assume that the third Doctor and a very bewildered Sarah Jane are battling Linx at roughly the same time that the fith Doctor and Nyssa are fending off the Rutans.
Timeline: between Time Reef / A Perfect World and The Eternal Summer
LogBook entry and TheatEar review by Earl Green
Review: A tongue-in-cheek trip into the past, Castle Of Fear can be taken only slightly more seriously than, say, the average episode of Black Adder. The chief difference is that Black Adder features very, very few alien shapeshifters trying to take over Earth, 
and in this instance, “very, very few” is a slightly euphemistic understatement for “none.”
Stockbridge itself has a storied history in Doctor Who expanded universe lore, having originated in the Doctor Who Magazine comics of the 1980s. Aside from the setting itself, nothing here connects to those comic stories, so a prior working knowledge of the DWM strips is not required to enjoy the story. A passing acquaintance with the Rutans might help, though the creatures are reintroduced and explained for those not familiar with them from their single prior TV appearance.
The humor and comedy of Castle Of Fear may be off-putting for those expecting unrelenting drama, but in many cases the juxtaposition helps the dramatic moments to stick out even more than they already do. The cast is excellent, and the end of part 4 by no means resolves the story – not by a long shot.