“Great Absence,” the festival-feted “breakout film” of director and co-writer Kei Chika-ura, is a somber, obscure mystery about memory, regret and entropy. It’s centered on a son trying to learn what his long-estranged father was really like before dementia took hold.

But the director of the engrossing, culture-dissecting “Complicity” makes his subtext Japanese culture itself — with an ageing populace, rigid social codes and adult children at a loss to understand how it call came undone in a single generation.

It opens with a SWAT team swarming around a tidy condo. An old man (Tasuya Fuji, who came to fame in 1976 with “In the Realm of the Senses”) opens the door, dressed and carrying his valise, seemingly resigned to his fate.

His son Takashi, played by Mirai Moriyama (“We Couldn’t Become Adults”) is an actor rehearsing a new multimedia play. He and wife Yuki (Yôko Maki of “After the Storm”) cross the country to deal with a social services review board planning the retired college professor’s care, and meet with the father who left his family, remarried and had little to do with the kid he nicknamed “Takkun” afterwards.

Our first impressions of Yohji Toyama (Fuji) aren’t pleasant. He’s bullying, brusque and “callous” his son admits. And he’s forgetting things, mixing-up facts. Rehearsals take a back seat as Takashi digs into the mystery that this man was and is.

His second wife, Naomi? He did marry her, right? Where is she?

“Dead,” the old man announces gravely. Cheated by electricians, raped. She killed herself. We, like Takashi, suspect that this isn’t true.

As Takashi and Yuki go through the house, he happens upon Naomi’s diary, with Yohji’s letters confessing decades-long romantic longing for her. The son decides that explains the loveless marriage he grew up under.

Every visit to his father deepens the mystery of his recent years. Is Naomi dead? Her son from her first marriage shows up, wanting help with her hospital care. Which hospital? Can we go and see her?

Um, no.

His father mentions an old colleague and protege who had his Dad lined-up to give a lecture and had no clue about his declining mental state.

And as these pieces of the puzzle come to light, the film’s point-of-view shifts, with flashbacks showing us Naomi’s plight, being married to an overbearing jerk who, as his mental faculties fail him, refused to give up taking the wheel or express what he should to her before his memory gave out.

A dementia-sufferer’s life of endless post-it notes, memory-prompt photo albums, lapses and their consequences is glimpsed in mostly quiet understated scenes.

Fuji’s performance is the highlight here, a man of science and obsessive Ham radio buff struggling to communicate what he’s going through but failing to soften his personality as his memory, and the self-control it might contain, fail.

Yohji lays out Japan’s “population loss” and the inevitable slide that accompanies it in a simple chat with his Ham radio gear salesman. The entropy here is personal, symbolic and grim. Who will take care of the rising tide of Yohjis in a culture that is literally shrinking, generation by generation?

The letters touch the son and the viewer in the contrast they provide — a judgemental grump who pined for the woman he finally would-up with in writing that is poetic and achingly romantic.

The shifts in points of view, slow-to-come explanations and slack pacing test one’s patience in ways “Complicity” — track that one down — never did. But Kei Chika-ura immerses us in these lives, in the sins of the fathers and the puzzlement of the children who recognize the “absence” of parents, even those who were never there for them all along.

Rating: unrated, implied violence

Cast: Mirai Moriyama, Tasuya Fuji, Yôko Maki and Hideko Hara

Credits: Directed by Kei Chika-ura, scripted by Kei Chika-ura and Keita Kumano. A Gaga release.

Running time: 2:13