PBS' "Masterpiece Mystery" is one of the joys of every summer. This year it's back with two favorite British detectives, Inspectors Lewis and Morse. Inspector Lewis, Series VI 8 p.m. Sundays, June 16-30, PBS Inspector Robbie Lewis (Kevin Whately, right in left photo) and DS James Hathaway (Laurence Fox, left) ponder their futures in three new feature-length episodes that find them investigating murders among the stones halls of Oxford. Endeavour, Series 1 8 p.m. Sundays, July 7-28, PBS After the success of its "Endeavour" pilot, "Masterpiece" brings back young Endeavour Morse (Shaun Evans, right) before he became and inspector. In four episodes set in 1965 Oxford, the detective constable rubs most of the force the wrong way with his unusual investigative techniques.