Know what they are about

In the passage below, I don't quite understand the last sentence of the last paragraph. Could you please rewrite it in terms which are easy for English learners?

Even so, from the moment Yuri arrives on the ferry, following a stunning opening shot of her passage through the picturesque Goto Islands, we know an uplifting change-of-heart is in her future. And so it goes, though the film begins as something of a teen comedy, with a gaggle of unruly boys signing up for the previously all-girl club after getting a look at its gorgeous new coach. The girls, led by the outspoken Nazuna (Yuri Tsunematsu), protest this barbarian invasion, but the bored Yuri shrugs off their complaints and coolly dismisses their dream of winning a national chorus contest.

From this prickly start the film descends toward the treacly, with everything from Yuri’s disastrous concert walkout to a gifted boy singer’s loving relationship with his mentally disabled brother used to pluck heartstrings. But the kids are fresh-faced naturals and Aragaki, who only a few short years ago was playing sweet-teen roles herself, gives a bracingly astringent performance as Yuri. Her smiles, when they arrive on schedule, blaze through the film’s teary mist. Those boys, we realize, knew what they were about.