Worth watching

8 p.m. (Ch. 13) Last Tango in Halifax

Season two of the seriocomic British series begins with “Episode 1,” as Celia and Alan (Anne Reid, Derek Jacobi) map out their future after being reunited. They’re in a relatively happy situation; not so with Gillian (Nicola Walker), whose emotions become increasingly frayed.

Not very long after its second season wrapped up, the mystery series’ season three begins with “New Hundred.” Carrie and Al’s (Poppy Montgomery, Dylan Walsh) murder probe leads to their working alongside the Secret Service. Evidently, the homicide is connected to a counterfeiting operation run by someone very lethal. Boris Kodjoe (“Soul Food”) guest-stars .

10 p.m. (Ch. 2) Unforgettable

In the premiere of this comedy series, British acting legends Ian McKellen and Derek Jacobi team as longtime companions who regularly trade barbs. The first episode finds them hosting a gathering to mark the passing of a friend. Such an occasion might be quiet and gloomy — but not in the company of Freddie (McKellen) and Stuart (Jacobi), whose spirited bantering transforms the event into something else entirely.

10:30 p.m. (Ch. 13) Vicious