An entertainment era has ended. Over a 24-hour period, we’ll see the final episodes of “Lost,” “24” (7 p.m., Fox) and “Law & Order” (9 p.m., NBC), each of them a defining and important series in the history of television.

Of the three, only “Lost” leaves the stage with viewers still clamoring for more. “Law & Order” will be missed, but with three surviving spinoffs and hundreds of repeats to watch, “Law & Order” has weathered the harshest TV adversary of all — competing against itself. And it has done so for 20 years.

Tonight’s “Law” unfolds like any other and not terribly much like a chapter-closer. A torn-from-headlines case ricochets from an Internet creep to a bomb threat not unlike the recent incident in Times Square to a strange tale of “Rubber Rooms” where teachers are warehoused — at full pay — while awaiting disciplinary action. That last story “headline” came via a story in The New Yorker magazine and not the New York Post, but never mind. “Law & Order” remains current to the end.

• Over eight seasons “24” burned hotter, brighter and louder than many other series until it just burnt out altogether. But along the way it managed to be something rather remarkable — a gripping series with comic book plotting and characters, hyper-violence and cliff hangers straight out of the silent-movie era that at the same time managed to seem frighteningly relevant.

The pilot for “24” had been produced and promoted before 9/11 attacks. Yet almost from the start, the series seemed to define and inform serious conversations about torture and the limits of the law in the face of terrorism. “What would Jack Bauer Do?” became both a question and a slogan. But we all knew Jack Bauer could do anything. But at what cost?

At the end of the day, or rather eight days, Jack Bauer’s most enduring bond was not with the president or CTU big-shots but with the nerd who has had his back (and managed his firewall) all along. It’s probably time for Jack Bauer to hang it up. But I’ll miss Chloe O’Brien.

Tonight’s series finales

• Marshall and Lily make a pact on “How I Met Your Mother” (7 p.m., CBS).

• The Ring tightens on “Chuck” (7 p.m., NBC).

• A surprise wedding on “Rules of Engagement” (7:30 p.m., CBS).

• Charlie loses his license on “Two and a Half Men” (8 p.m., CBS).

• Sheldon gets dragged into the online dating scene on “The Big Bang Theory” (8:30 p.m., CBS). “Bang” moves to Thursdays in the fall.

• A killer taunts authorities in secret code on “CSI: Miami” (9 p.m., CBS), moving to Sundays next fall.