Boreas, maybe it's a common subject because it allows scope for examination of reactions to it. The kidnap victim almost joked about gang-rape, as if a way for her to cope with it. That's difficult to do without trivialising it, shrugging it off, yet it seemed okay.
But I didn't like Catherine's son talking about his sister having been "asking for it". It's credible dialogue for a jealous brother, but dangerous territory.
I thought it was a great production. Lighter moments in some of the other episodes of police incidents for Catherine to deal with but there were no wasted scenes.
On the question of unpleasant themes for drama, I am always fascinated by the incidence of murder, from Agatha Christie to Morse, the early Taggart's catchphrase "there's been a murder", and especially the light-weight Murder She Wrote. The taking of precious life is as serious as it gets, and unfortunately there's too much of it on the news in real life, yet it is the essential subject matter of a branch entertainment. Curious?