Despite its faults, there are two consistent sources of pleasure in “Damsel.” The first is that it starts off a fairy tale and turns into a monster movie, a kind of “Cinderella” meets “Beowulf.” That combination is so ridiculous and insane that the filmmakers really deserve to be congratulated.

The other source of mirth is how Millie Bob by Brown attacks the central role — “attack” really is the right word — with an intensity worthy of Kate Winslet or Cate Blanchett going for another Oscar. When the dragon (yes, there’s a dragon) puts his huge claw on top of her, Brown is ready for it, channeling the collective pain and outrage of every person who ever got stepped on by a monster.

The humor here — unintentional, of course — comes from the clash between Brown’s ferocious commitment and the absolute unworthiness of what she’s committing to. Yet that commitment is appealing and has force behind it. Like a ballplayer running full-out to first base even though he knows he’s going to get thrown out, Brown is just doing what she’s supposed to be doing.

She plays Elodie, the princess of a kingdom from long ago and far away, whose father, the king (Ray Winstone), contracts for her to marry a prince. It’s a good deal on paper. Basically, Elodie’s kingdom is a dump, like one big medieval parking lot, if such a thing can be imagined. But the nearby kingdom is wealthy and opulent. So Elodie is definitely going to live well.

What could go wrong? The prince (Nick Robinson) seems like a nice guy, and he isn’t bad looking, either. The only indication of trouble is that the queen, who is played by Robin Wright, does a lot of smiling, but it’s the smile of someone who can’t stop sizing people up or measuring them for a coffin.

The young couple has a traditional wedding service, and that goes well. But then there’s a second ceremony, a welcoming of Elodie into the family, and that one doesn’t go well, not at all. One expects a certain social discomfort with these rituals, but no one expects them to end with a fire-breathing dragon chasing a princess. That’s a little extreme.

And from then on, that’s the gist of the whole movie, which is about little besides Brown fleeing a dragon or acting her heart out in front of a CGI screen. At least half the movie involves the princess’ struggle for survival, except it takes place against a fairy-tale background, so we’re not exactly worried about her.

The apportionment of time in “Damsel” seems out of balance. The movie could have used a little less of Brown cowering behind boulders as the flames get closer and a little more of her getting even with the people who put her in this predicament. Director Juan Carlos Fresnadillo certainly seems willing to go hardcore with the dragon sequences. He should have also lavished time and horror on the scenes of Elodie getting even.

Still, there’s one unalloyed good thing to be said for “Damsel”: It marks the end of Millie Bobby Brown’s apprenticeship. Her child actress years are over. She’s grown up and ready to star in movies that audiences can take as seriously as she does.

 Reach Mick LaSalle: mlasalle@sfchronicle.com