TONIGHT'S MUST-SEE

"Masterpiece: The Last Enemy," 8 p.m., PBS. Two brothers grew up as opposites. Stephen, worried and withdrawn, became a math genius. Michael, open and zestful, aided refugees in Afghanistan. Now, hearing that his brother was killed, Stephen returns from years in China. He finds London filled with paranoia. He has new worries of his own. Who is the strange woman in Michael's apartment? And the dying woman in Michael's bed? And who is the silent stranger stalking them? Don't expect quick answers. The stranger, for instance, doesn't speak a word tonight. Mysteries and deceptions tangle. Don't start this unless you plan to watch diligently for five Sundays. If so, you'll catch a bracing, intelligent thriller.

"Desperate Housewives," 8 p.m., ABC. Mike finally meets Susan's young lover. Meanwhile, neighbors remain icy to Edie, while trying to learn more about her husband (Neil McDonough).

OF NOTE

"The Simpsons," 8 p.m., Fox. Dreams differ: Bart obsesses about getting a cell phone; Lisa reaches Peru's Machu Picchu.

"Cold Case," 8 p.m., CBS. In 1991, it was believed that an inner-city teacher died in a carjacking gone bad. Now murder is suspected.

"True Blood," 8:30 p.m., HBO. After some rough hours, here's a gentler one. Bill gives a talk about his time as a Civil War soldier. Also, flashbacks show how he became a vampire.

"The Unit," 9 p.m., CBS. After an assassination plan was barely stifled last week, team members left the base and their families were whisked to a secret location. Tonight, the team tries to protect a scientist on a commercial airline.