Having last year put a toe in the water with an exploratory pilot and found it fine, "Endeavour" returns to the PBS series "Masterpiece Mystery" Sunday with four new episodes. They are excellent company, even if they sometimes feel too coincidental, complicated, clever or corpse-strewn to be true.

The series is a prequel to the beloved "Inspector Morse" (1987-2000), which starred the late John Thaw as an Oxford-based police detective. (The character's first name provides the new show's title.) While Kevin Whately, who played his old number two, mans the ongoing timeline in the sequel "Inspector Lewis," "Endeavour" jumps back to the mid-'60s, when the future chief inspector was yet a mere detective constable; it keeps the character alive at the same time it feeds our not wholly exhausted taste for midcentury period drama.

As in the pilot, the backbone of the series is the relationship between young Morse (Shaun Evans) and his immediate superior, Detective Inspector Fred Thursday, wonderfully played by Roger Allam, whose manner, shape and dark-stout voice (the series' pedal tone, to venture a musical metaphor) recall Michael Gambon in both "The Singing Detective" and "Maigret." And like Maigret, he has a pipe.

PHOTOS: Hollywod backlot moments

We get to know Thursday better this year: In one lovely scene, Morse sits down with the family at breakfast, which makes him a little uncomfortable and a little happy; we know that his own life will never look like this.

Like a milder, slightly more flexible version of his irascible later self, Morse is socially awkward and sometimes abrasive — "a bloody misfit," someone calls him. Fortunately, the character exists in a time when no one diagnoses him as "a little autistic."

He is already addicted to puzzles and opera and displaying a signature weakness for distressed damsels: "When it comes to a bird with a wing down," observes Thursday, "you've got a blind spot a mile wide." But he is also chivalrous (or shy): When one mixed-up young woman implores him, "Take me to bed," he reads her to sleep with poetry instead. (He is literary as well as musical.)

Morse knows that he's the smartest young buck in the herd, which naturally irritates his co-workers, of whom Det. Sgt. Peter Jakes (Jack Laskey) is his most immediate and vocal rival. At the same time, there is a moment in each film where he smacks his forehead and calls himself an idiot, having missed some crucial clue that has been staring him in the face all along.

PHOTOS: Celebrities by The Times

Russell Lewis, who also developed "Inspector Lewis," wrote all four episodes, though different directors bring different quirks to the material. Some get a little too fancy with the framing and focus, and the staging of certain Tense Moments relied more on movie convention than the show's own aesthetic logic. (They struck me as funny in a way I am sure was not intended.) The unfussy approach that worked for "Morse" also works best for "Endeavour." Still, if there is more action than in the parent series, "more" in this case does not mean "much," and things stay mostly slow, quiet and a little mournful. Evans does not attempt a John Thaw imitation, but he sits deep in the part.

The new season adds Anton Lesser as a flinty new boss for both Thursday and Morse, who warns the station house against "breaches of procedure or Spanish practices" (a now-controversial term for workplace customs that cut labor a break); he bumps Morse back from the head of the class, to which Thursday has skipped him. Also new is Sean Rigby as friendly Police Constable Strange, who will have made chief superintendent by the time of "Inspector Morse."

robert.lloyd@latimes.com

--

"Masterpiece Mystery: Endeavour"

Where: KOCE

When: 9 p.m. Sunday

Rating: TV-PG (may be unsuitable for young children)