This isn't really a bad story, its just not very engaging. Six-parters often have a lot of filler, but this one is sluggish to the point of tedious. There isn't a lot of drama, excitement, or anything to really keep the viewer interested in "what happens next." Even the actors are rather dull. Yes...that even includes future Romana Lalla Ward as Princess Astra.
A couple of quick trivia notes...this is the last time that K9 is voiced by John Leeson until he returns in season 18, and the last to feature Mary Tamm as Romana...and it is she who recommends that Lalla replace her.
