TONIGHT'S MUST-SEE

"The Class," 7:30 p.m., CBS. In its first season, this show has become TV's best comedy. Now it has its final new episode — and a need to be discovered quickly. Next week, "The New Adventures of Old Christine" returns and "Class" rests wondering if it will have a second season. Strong ratings tonight would help. All season, this show has nudged the sunny Ethan (Jason Ritter) and the cynical Kat (Lizzy Kaplan). It has also prepared Nicole to run off with Duncan. Now both stories are at key points, just as Yonk (the decent dolt married to Nicole) is in the hospital with an apparent heart attack.

"The Black Donnellys," 9 p.m., NBC. Last week's sensational opener ended with Tommy Donnelly defending his brothers the hard way. He killed the local leaders of the Italian and Irish mobs. Things don't get easier now. Sean's in the hospital, Jimmy's in jail, Tommy and Kevin have major cover-up work to do. It's another great hour, with subtly deep portraits of the women. Catch Kate Mulgrew as the Donnelly mom and great closing scenes with Olivia Wilde as Tommy's eternal (and married) love.

OF NOTE

"Barbarians II: The Vandals," 6 p.m., History Channel. With their control wobbling, British leaders and Roman colonialists took big gambles: They invited fierce tribesmen to help them fight. These were dangerous steps, as we'll see in the first couple nights of this four-night series. Punctuated with lots of re-enacted battles, the documentary shows how a handful of invited warriors grew into a ruling mob.

"Prison Break," 7 p.m., Fox. Michael encounters the crooked president (Patricia Wettig) who started this. Also, Fox promises we'll learn what's on the tape that Michael is pinning his hopes on.

"How I Met Your Mother," 7 p.m., CBS. In a rerun, Barney temporarily shares his apartment with Lily; clashes begin.

"Heroes," 8 p.m., NBC. The new hero, Candice, is played by Missy Peregrym, a preacher's kid from Canada who previously starred in "Black Sash" and "Life As We Know It." Meanwhile, Hiro's search for the sword takes a dark turn and Isaac paints his fiercest vision. We also get a look at the new "Spider-Man" film in a minute-long clip during the show.

"24," 8 p.m., Fox. Logan, who was the shaky president last season, offers to help Jack. Understandably, Jack is wary.

"Rules of Engagement," 8:30 p.m., CBS. Russell (David Spade) hooks up with an older woman whom he disappointed in bed on his 18th birthday.

"What About Brian," 9 p.m., ABC. In a fairly good episode, Brian finds that lots of women — the wrong ones — are drawn to him. Meanwhile, Adam is finally ready to end his temporary celibacy - but has no one to end it with.