Bob Morley as Bellamy and Marie Avgeropoulos as Octavia in “The 100.” (Cate Cameron/CW)

In the CW’s unexpectedly excellent new sci-fi series “The 100” (premiering Wednesday night), Earth was torched in a nuclear war 97 years earlier. Probably Vladi­mir Putin’s fault.

But here’s a silver lining: The millennials are all dead! They either were annihilated or died of old age on a last-resort orbiting space station called the Ark. Gone with them is a certain millennial inertia that seeps into so many of our TV shows; here at last is an action-adventure series that is about desperate, futuristic teenagers and young adults who aren’t burdened with questions of demographic identity or hopelessly drifting between a niche and hard place. They’ve just got the hard place.

Granted, the 100 stranded young’ns of “The 100” aren’t model citizens (they are reprobates and delinquents, banished from the Ark and sent down to Earth to serve as lab rats), but instead of putting “leadership skills” on their résumés, they’re putting said skills into action, even if some of them choose a “Lord of the Flies”-style management objective. The very first thing they do? Cut off all communication with their parents, disabling the life-meters that let the grown-ups on the Ark above monitor their vital signs.

“The 100” is a fine example of how a TV show can exceed its premise and slightly transcend the junk that its network typically serves up. It’s also a neat lesson for the TV critic: Keep an open mind, because you never know where the next pleasant surprise is coming from.

Now, don’t get too excited. No one is going to mistake “The 100” for SyFy’s much-missed “Battlestar Galactica” or ABC’s now-classic “Lost,” and this is the CW, you know — home of bratty vampires and trophy-kid aliens. But going by the six episodes I’ve seen, “The 100” comes off as a smartly written and well-acted endeavor.

Based on a new series of novels, “The 100” mines a potent and lasting fantasy, in which the children are better equipped to deal with crisis than their elders. Four hundred people survived Armageddon by fleeing to the Ark. Now, three or four generations later, the Ark’s inhabitants number 4,000-plus. The oxygenator can’t keep up; their days are numbered.

Against the fretful counsel of the ship’s best scientist and doctor (Paige Turco as Abby), the Ark’s leadership (including a would-be tyrant, played by Henry Ian Cusick) votes to jettison 100 jailed youths down to the deceptively bluish-green planet below, to see if it’s survivable. Among those forced onto the shuttle is Abby’s daughter, Clarke (Eliza Taylor).

The kids’ rickety shuttle is spat out toward a forested spot in North America. After a rough landing, the Hundred (as they come to be called) discover a sylvan paradise awaiting them. The fresh air! The trees, the babbling brooks! The mutated fauna with two heads! And we shall call it Portlandia.

While most of the Hundred decide to follow the head bully (Bob Morley) and set about celebrating their sweet freedom, Clarke and the more survival-minded kids try to reestablish radio contact with the Ark. On an expedition to find the remains of a secret war bunker, they instead encounter terrifying evidence that Earth is still very much inhabited by humans — or something human-like.

What I enjoy about “The 100” is its serious sense of urgency, which is evident not only in the plot but also in the producers’ wise instinct that nobody gets to waste time when trying to sell today’s scattered TV audience on another sci-fi soap opera.

There are, of course, those trite moments when these teens stamp their feet and deliver supercilious monologues to one another about working together, when they’re not falling into love triangles. That comes with the territory in CW land.

Is it a fantastic TV show? Hmm. Let me just say this: I popped “The 100” into my DVD player and braced myself for another round of midseason torment. Several episodes later, I had that weird, happy feeling one gets from being simply and satisfyingly entertained.

The 100

(one hour) premieres Wednesday

at 9 p.m. on the CW