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.

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