Though it is probably the most popular Latter-day Saint teen romance novel of all time, Jack Weyland’s “Charly” is nonetheless a very bad book. It attempts to compress a lengthy story spanning several years into 100 pages, rushing through every conflict like a videotape on fast-forward. Characters behave irrationally and change suddenly, all in the apparent interest of getting the whole thing over with in less time than most books choose to allot themselves.

Its potential as a movie, then, was considerable: Films are better equipped than books to squeeze a lot of information into a short space. Plenty of movies successfully depict characters’ entire lives and still occupy no more than a couple hours; books usually need a few hundred pages to accomplish that.

Directed with great competence and compassion by first-timer Adam Thomas Anderegg, the film version of the book (adapted by Janine Whetten Gilbert) is true to Weyland’s story while expanding and improving it.

Charlene “Charly” Riley (Heather Beers) is a fun-loving girl visiting family in Salt Lake City. She is set up with — foisted upon, perhaps — Sam Roberts (Jeremy Elliott), a strait-laced, humorless Mormon lad. They are destined to fall in love, of course — but she has a fiancé (Adam Johnson) back in New York, and her parents (Gary Neilson and Lisa McCammon) are fearful of her increasing interest in Sam and his Mormonism.

That’s the first half of the movie, the happy half. Then comes the weepy half. The book tells you the outcome up front, but the film doesn’t, so I won’t say any more.

The film stock is good; the cinematography is nice; the supporting cast is strong; the production values all indicate professionalism and talent. But the movie’s greatest asset and liability are its two stars.

As Charly, newcomer Heather Beers is splendid. I don’t even like Charly in the beginning — she’s too flighty and single-mindedly free-spirited — yet I like Beers in spite of my issues with her character. She has charm, energy and wit.

“Charly” earns the audience’s tears at the end, rather than forcing them out of us, because of Beers’ commitment and skill level. By the end, she has deftly woven maturity and gravity into Charly’s persona.

Her co-star, Jeremy Elliott — in his third LDS film this year — does not fare as well. In his first scene, he seems relatively normal. Almost instantly, in the next scene, he is nerdy and uptight. He overdoes the “repressed Mormon boy” bit to the point that he becomes one-dimensional.

As a result, the film’s third act — which should be about whether faith has practical applications in real life — falters, failing to completely address the issues it needs to. Elliott’s performance simply isn’t compelling enough to shoulder the weight of the film without Beers next to him. His soon-to-be-infamous “breakdown” scene, which involves throwing a lot of paint around, is one of the least convincing moments in recent memory.

The movie’s sentiments are lovely, and for the most part they are expressed well. To the extent that Mormon doctrine is present, it is delivered without much sermonizing. It is a likable story, told with competence.

B- (1 hr., 44 min.; PG, some very mild sexual innuendo, thematic elements.)