Maisie Williams is the spark and spunk, the light and the dark of “Then Came You,” a montage-happy “terminal teenager” romance with enough twists and turns, laughs and near-tears to make it work.

Williams, of “Game of Thrones,” has to do the heavy lifting because her co-star is Asa Butterfield, over ten years into his career, as pretty as ever, and unfortunately just as bland.

Director Peter Hutchings of “The Outcasts” cleverly picks up some of Butterfield’s slack with musical montages  — two young people getting to know one another, enjoying a party together, one of them taking his first flight, the other — with his help — polishing off her “To Die List” of things she wants to accomplish before shedding her mortal coil — set to sensitive, bouncy pop.

The rest he leaves to Williams, and she doesn’t disappoint.

Skye is a Brit living in Albany, and we meet the 17 year-old as she’s getting the worst news you can get. “The tumor hasn’t responded to treatment,” her doctor tells her and her parents. The kid accepts the gravity of the situation, but she cannot change who she is.

“You win some, you lose some.”

She meets brooding loner Calvin (Butterfield) at a cancer support group. He’s a college drop-out who works with his dad and brother (David Koechner and Tyler Hoechlin) as a baggage handler at the local airport. He never sees what hit him.

Skye interrupts his scribbling an answer to the group’s sort of “fondest wish” exercise.

Don’t write “going to Disneyland,” she confides. “It sucks and there’s like thousands of ‘dying kids’ there,”  so you’re not special.

Better idea? Write “Ask whatshername on a date.”

Or steal one of the other items on Skye’s “To Die List” — “Fight fire, Perform Shakespeare, Vanquish a Foe, Win an eating contest (Nope, she’s already done that and x’ed it out), Demolish a Car, Get a Job, get Fired, Get Arrested.”

Whatever resistence Calvin shows to falling under this girl’s spell, we KNOW they’re going to connect and by cracky he’s going to help her with that list.

“Don’t worry. I’m not going to DRAG you to Amsterdam!”

Yeah, she’s seen “The Fault in Our Stars.”

The list? It’s handled with one bit of shoplifting and a cutesie musical montage of most of the rest, sparing us the Teen Take on “The Bucket List.”

Still, the shoplifting leads to a running gag. Ken Jeong plays a cop whose father is in the cancer support group, Briana Venskus his “F— cancer!” partner. They let these “dying kids” get away with a LOT and even pitch in on a few “list” items.

One problem with all that? Calvin isn’t dying. He says “I am NOT A hypochondriac!” but he pretty much is. He keeps a “My Symptoms Journal.” No, you don’t want to be in the waiting room with him when he thinks he has testicular cancer.

He was in the support group because his frustrated doctor wants him to “Get a little perspective.”

We all know that perspective will from Skye. But “Whatshername” turns out to be a pretty, lonely stewardess with a regional airline (Nina Dobrev).  He’s younger than her and no matter how much her fellow flight attendant (Tituss Burgess) wants her to do something about that, it takes Skye’s interference to move that ball down the field.

“You have a lot of living to do, Calvin Lewis.”

As familiar as the path “Then Came You” generally takes might be, it’s got lots of clever laugh-at-death touches, a few sparkling surprises and a gut-punch third act “reveal.”

Skye brings Calvin a gift as she is reaching out to him, a goldfish in an IV bag (hanging from a drip stand, no less). She dresses as ‘Death’ for a costume party.  Williams makes this pixie, as such pixies often are in movies with terminal teens, a force of nature — a lifeforce.

“But I’m dying” is her comeback for anything she’s denied. And there’s no point in Calvin trying to get his “big confession” out. she just bowls him over with words blurted out in her “no time left to lose” rush.

Her mania makes “Then Came You” play lighter and go down easier than “The Fault in Our Stars.” But that’s a shortcoming, too. “Fault” and many other movies of this sub-genre work when they tear tears out of us, and “Then Came You” just deflates big emotions.

It could use a few more of them, to be honest.

Butterfield sets off no real sparks with screen veteran Dobrev (“Vampire Diaries,” “The Perks of Being a Wallflower”). She’s not up to the heroic task of turning “The Space Between Us” star into an object of desire, or even romantic curiosity.

Perhaps Netflix’s “Sex Education” series will do what none of his big screen romances have managed.

But quirky Brit girl next door Williams is the reason to see this, and she’s as delightful as the role demands — a perky, funny, sweet and romantic kid that we’ll be sad to see go when the inevitable happens, as it does in all terminal teen romances.

MPAA Rating: unrated, sexual situations, profanity

Cast: Maisie Williams, Asa Buttefield, Nina Dobrev, Peyton List, Ken Jeong, Tituss Burgess, David Koechner

Credits: Directed by Peter Hutchings, script by Fergal Rock. A Shout! Factory release.

Running time: 1:37