“Chuck” (7 p.m., NBC) has morphed over its short run from being about a slacker who becomes an inadvertent spy who’s baby-sat by a beauty, to Chuck as a real spy with a real romantic chance with his former minder.

Tonight’s “Chuck” has Chuck (Zachary Levi) and Sarah (Yvonne Strahovski) in Europe, on their first foreign assignment together as a couple. And along the way, they discover strangers on the train who just might be Spanish spies.

Will viewers buy Chuck and Sarah as lovers? If the ratings are any indication, they don’t. And if TV history is any guide, this means “Chuck” is entering its twilight, the time when Maxwell Smart and 99 settled down on “Get Smart” or when “Moonlighting” stopped teasing us about the tension between its leads. Only, “Chuck,” for many reasons, never got quite as popular as those two series. And its chances for renewal are still very much up in the air.

• “Dancing with the Stars” (7 p.m., ABC) continues without its train-wreck star Kate Gosselin, a poster child for reality television’s inverse proportion of vulgar ego to actual talent. Imagine asking her to talk about something besides herself? Not easy, is it? Now viewers will actually have to focus on the dancing.

• “American Experience” (8 p.m., PBS, check local listings) looks back at the 1968 My Lai massacre, one of the bitterest chapters of the long sad saga of the Vietnam War. “My Lai” features extensive interviews with Aubrey Daniel who prosecuted Lt. William Calley, who became a touchstone of “culture war” politics during the Nixon years. It also features conversations with men in Calley’s “Charlie Company” who did not take place in the mass murder as well as Vietnamese survivors of the massacre.

Tonight’s other highlights

• A wandering wife (Sarah Wayne Callies, “Prison Break”) suffers a mysterious ailment on “House” (7 p.m., Fox).

• A failure to communicate on “Trauma” (8 p.m., NBC).

• The contest to become a high school’s official spokesperson knows no bounds in the 2010 kids comedy “Harriet The Spy: Blog Wars” (7 p.m., Disney).

• Taylor continues her diplomatic offensive on “24” (8 p.m., Fox).

• No more Mr. Nice Guy on “Romantically Challenged” (8:30 p.m., ABC).

• The late Paul Winfield narrates a “City Confidential” (9 p.m., Biography) from 2004, “Honesdale: Highway to Hell” a tale of a high school valedictorian’s death that leads to murder in a small town on the outskirts of the Poconos.

Cult choice

• The 2005 documentary “Murderball” (7 p.m., Sundance) follows a league of paraplegic rugby players.