In an effort to save his sister from a North Korean labor camp, Myung-hoon (Choi Seung-hyun) goes undercover as a South Korean high school student in “Commitment.” (Well Go USA)

A boy-band star plays a conflicted antihero in “Commitment,” one of the more conventional Korean movies to get a U.S. release in recent years. That’s conventional by fatalistic East Asian standards, however, not sunnier Hollywood ones. Beliebers would be scandalized if their idol turned up in a drama this grim.

The story begins with a North Korean spy’s farewell to his handler in the South. The agent thinks he’s going home, unaware that the ruthless Pyongyang regime doesn’t allow its operatives to repatriate. Instead, they’re killed.

That would be useful information for Myung-hoon, the dead man’s son. He’s been forced to accept an undercover mission to the South, in the hope of securing his little sister’s release from a labor camp. Myung-hoon is played by Choi Seung-hyun, who started his career with the group Bigbang under the K-rap name “T.O.P”. He’s 26, but doesn’t look out of place in his character’s new habitat: a South Korean high school.

Discipline is lax at the school; the newcomer is regularly menaced and soon is protecting Hye-in (Han Ye-ri), a pretty classmate. She’s an orphan and an aspiring dancer, which exposes her to, of all things, ballet bullies. Hye-in also has the same name as Myung-hoon’s sister.

When not using his martial-arts skills in school hallways and bathrooms, Myung-hoon employs them as a hit man. His targets are not South Korean. The movie supposes that a North Korean power struggle is playing out south of the border, an ideological conflict that’s reduced to government designations: Myung-hoon works for Unit 8, his foes for Section 35.

A bureaucratic rivalry that turns to war. Pampered high-school brats whose victim turns out to be a trained assassin. A sister and a potential girlfriend with the same name. Any or all of these scenarios might merit an ironic wink, but “Commitment” never cracks a smile. It’s as stern as Choi’s single-expressioned performance.

Having ruled out humor, the movie emphasizes action and melodrama. Director Park Hong-soo, making his feature debut, handles the former with proficiency but little flair. Anyone who expects the baroque bloodletting of Korean directors like Park Chan-wook (the original “Oldboy”) will find the movie competent but tame.

The filmmakers are committed to tragic romanticism. If they muster few thrills and no surprises, at least they don’t stoop to a happy ending. For Choi’s fans, “Commitment” should provide a good cry. Even if its script would have benefited from a few laughs.

Jenkins is a freelance writer.

★ ★

Unrated. At Cinemark Egyptian 24 and XD and Rave Cinemas Centreville 12. Contains bloody violence and occasional profanity. In Korean with subtitles. 113 minutes.