The PBS series Masterpiece Mystery returns with another year of relatively thoughtful genre stories tonight, starting with a new Lewis mystery titled “Down Among the Fearful.”

The series, featuring characters from Colin Dexter’s “Inspector Morse” novels and short stories and the Inspector Morse TV series that was based on them, tends to reside on the more thoughtful and less trendy side of the Masterpiece spectrum. Detective Inspector Robbie Lewis, formerly the sergeant assisting the now-deceased Inspector Morse, is an intelligent, sensible, mature, skeptical, and practical but not very imaginative or intellectual detective—just what one would have expected based on the character in the Morse series. He’s paired with Detective Sergeant James Hathaway, a former divinity student who retains his religious faith and is wont to read heavy books and ponder deep questions.

This week’s case allows both of the a good opportunity to show their personalities and interests. The two investigate a series of murders centered on a phony psychic who works with grief-stricken people, and on a psychology experiment in which the scholars break down people’s religious beliefs using “logic and reason,” as the psychologist in charge tendentiously puts it. The experimenters “grill” and “mock” the religious-believer subjects of the experiments, as one of the subjects accurately characterizes it, and they seem to be vastly more interested in giving themselves reasons to feel good about their atheism than to understand what religious faith really is and does. The experiment is ultimately suggested to be nothing more than a cruel and idiotic exercise in brainwashing.

The Oxford professor conducting the study does not make an attractive advocate for atheism: he is arrogant, cold-hearted, and openly contemptuous of nonintellectuals. The episode does not suggest that religious believers are without shortcomings of their own, however, as the unfolding of the mystery shows. Nonetheless, it is the professor’s “experiment” that sets the killings in motion.

Interestingly, the series is increasingly bringing Lewis’s partner, Hathaway, to the fore, with Lewis even becoming something of a supporting character. In so doing, it moves the show more toward the tone and interests of Inspector Morse, the series from which Lewis is derived, while reversing the ages and departmental status of the two. Morse was the central character and the more philosophical and deep-thinking of the two in the earlier series, and Lewis was his callow partner. Hathaway fills Morse’s role now, despite his inferior departmental status, and his greater prominence in recent seasons makes the show increasingly interesting and sophisticated.

This is particularly important to the subject matter of the present episode, for middle-aged Lewis is an atheist and not interested in deep things such as arguments about theology, whereas Hathaway is a very thoughtful Catholic who studied for the priesthood before leaving school to join the Oxford police. As all of this suggests, Lewis is after more than just identifying the murderer, as it explores topics of deep import without ever becoming pretentious or lugubrious. Those who are interested in thinking a bit about the deeper mysteries of life while enjoying a cracking good story would do well to look in on Lewis.