Just when a viewer is lulled into believing “The Attorney” is a lighthearted comedy about a doofus of a lawyer, this powerful South Korean feature takes a sharp turn toward brutal political thriller.

Based on real events that took place in the nation following a 1979 coup d’état — resulting in martial law, totalitarian measures and a witch hunt for alleged communists — the film centers on the moving conversion of an affable opportunist to anti-authoritarian crusader.

Song (Song Kang-ho) is a high-school-educated lawyer whose only goal is to make money by taking on menial legal tasks most self-respecting attorneys ignore.

In a memorable scene that feels like something from a Jack Lemmon movie, Song listens quietly to other lawyers — unaware of who he is — as they bad-mouth him for profiting from minor chores. Song shrugs it off and hands them his business cards.

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What we don’t know during “The Attorney’s” first hour is that behind Song’s cheerful self-absorption, an entire country has suddenly come under the control of national security extremists. That reality hits home when a teenager, Jin-woo (Lim Si-wan) — with whom Song has a familial relationship — is arrested, tortured and put on trial for reading seditious literature.

“The Attorney’s” second half finds Song springing to action as Jin-woo’s counsel, taking on the entire government in an intense courtroom drama.

Co-writer and first-time director Yang Woo-seok — adapting the story from a web-toon he created — relies on audience suspension of disbelief in a show trial that would allow Song lots of leeway to rail against corrupt power.

It might seem improbable, but it’s sure fun to watch.

Tom Keogh: tomwkeogh@yahoo.com