It goes down easier if you remember you’re allowed to laugh.

Many of the best film noirs have their darkly humorous moments, and scenes and sequences so genre-iconic that a fan will give in to snickers of delight.

Director Park Chan-wook (“Stoker,” “Oldboy,””The Handmaiden”) goes full-on Korean noir with “Decision to Leave,” a delicious femme fatale thriller with mystery, tragedy and more than a few deadpan laughs.

Tang Wei (“Blackhat”) turns out to be a perfectly fatal femme in this whodunit/how-he-or-she-done-it, a beguiling suspect in a murder case being investigated by the intrepid but insomniac Det. Hae-jun, played by Park Hae-il of “The Host.”

Our lawman is a famously obsessive homicide detective in Busan, a man who lets his unsolved cases eat away at him so much he’s forever dozing off at the wheel and emptying bottles of eyedrops to stay awake on the job. Nights offer him no peace, as he has grisly murder-scene photos covering one wall of his city apartment.

He’s in a “weekend marriage” with Ahn Jeo-jong (Jung Yi-seo), who works at a nuclear power plant on the other coast of Korea, staying over with her in Ipo on Saturdays and Sundays. But as he notes, “People don’t stop murdering on weekends” (in Korean with English subtitles), so there’s no getting away from work.

At least this new case has “open and shut” written all over it. A rock climber fell from his favorite peak, videoing his trip as he did. Det. Hae-jun and his younger partner Soon-wan (Go Kyung-Pyo) go over the gruesome, fly-decorated corpse, track the dead man’s last actions and question his younger Chinese widow Seo-rae (Tang). Something about her underreaction to all this gets their attention.

“He perished as he wished,” she says, apologizing for her Korean. Sometimes she uses a phone app voice translator for help. But the language barrier isn’t all that makes them suspicious.

And when it turns out there’s other evidence, the cat and mouse game begins.

Seo-rae is weirdly casual around this detective, and as he stakes her out, gets her DNA and questions her about some intimate injuries, we see the sleepy copper’s problem.

He’s becoming obsessed, and this seemingly timid elder-care nurse from China can see it and seems into it.

Park and his sometime co-writer Seo-kyeong Jeong (“Thirst,””The Handmaiden”) fold in other cases that the police are working on, wanted murderers pursued with a breathless, bracing foot-chase (filmed with a shaky shoulder cam) or cornered on a rooftop stand off.

The jokes come from Hae-jun’s fellow detectives noting the extra attention he’s giving Seo-rae, the fact that he orders her “the priciest” sushi and kimchi take-out mid-interrogation and Hae-jun’s reactions to everything they notice. Are they worried about him?

“Killing is like smoking. Only the first time is hard.”

“Decision to Leave” has some fine second act surprises and third act twists. And some of the fun here is the difference between Korean and American tech and policing. The cops have an electrical winch-mountain climbing assist. Hae-jun leans heavily on his smart watch for surveillance, audio recording his stake-out notes. He packs a chainmail glove for use when a bad guy pulls a knife on him.

Like any American police officer would go to that trouble.

Start to finish, Hae-il gives us the feeling that Hae-jun is just lost in this lovely suspect’s eyes, under her spell. Tang in turn plays up innocence and a lack of guile to him — for a while — and letting the viewer in on Seo-rae’s curiosity about her tormentor and ability to size him up.

It is her cagey, tough and sexy turn that gives the title its double and triple meaning and the viewer every reason to engage with this latest winner from Park Chan-wook.

Rating: unrated, violence, some profanity

Cast: Park Hae-il, Tang Wei, Jung Yi-seo, Go Kyung-Pyo

Credits: Directed by Park Chan-wook, scripted by Park Chan-wook and Seo-kyeong Jeong. A MUBI release.

Running time: 2:18