First published in 1963, Jakucho Setouchi’s “Natsu no Owari (The End of Summer)” was the “Fifty Shades of Grey” of its day: a best-selling novel written by a woman that viewed the unconventional love life of its 38-year-old heroine with the sort of matter-of-factness then considered daring. But the story, based on the author’s own experiences, was written with the plotless naturalism of the typical watakushi sho￣setsu (I novel), while its impact came less from its heroine’s sexual exploits and more from her absolute honesty.

Scriptwriter Takashi Ujita has included the book’s most dramatic bits, while indulging in relatively little creative tinkering with its structure. This faithfulness extends to the film’s period look, with Kumakiri and his staff venturing to Kakogawa in Hyogo Prefecture and Awajishima in the Inland Sea to capture the right Showa Era (1926-1989) atmosphere.

The film’s biggest departure from the original novel is the casting of 27-year-old Hikari Mitsushima as the middle-aged heroine, Tomoko. An actress who has excelled at playing everything from a kick-ass cult member in the Sion Sono black comedy “Ai no Mukidashi (Love Exposure)” to the dying wife of an impoverished samurai in the Takashi Miike period drama “Ichimei (Hara-Kiri: Death of a Samurai),” Mitsushima gives exactly the performance the role calls for: emotionally transparent and complexly introspective. She inhabits Tomoko so naturally and vividly that we can read even her thoughts from moment to moment.

The film’s biggest problem, however, is that the youthful, delicate-featured Mitsushima does not resemble a woman in her late 30s who, in the novel, is imagining the end of her physical attractiveness to men. So what seems well-motivated on the page, from Tomoko’s eight-year affair with a seedy middle-aged novelist to her fling with a moody younger lover that offers her a last chance at sexual passion, looks somewhat strained, if not inexplicable, on the screen.

Set in the Showa 30s (1955-1964), the film begins with Tomoko’s return to the house she shares with Shingo Kosugi (Kaoru Kobayashi), her married lover, who has been going back and forth between Tomoko and his wife and child for years. This double life, which has become as regular as a train schedule, is calmly accepted by both Shingo and, for less obvious reasons, Tomoko.

An accomplished dyer, who is shown practicing her craft in scene after gorgeously photographed scene, Tomoko has no need of financial support from a man — and certainly not one like Shingo, who only goes through the motions of writing for a living. Also, whatever erotic charge that once existed in their relationship has vanished. But Tomoko feels comfortable and compatible with Shingo, despite his hangdog air, as he does with her, despite the lack of a marriage license. Why fix what’s not broken?

Then Shingo tells her that an old mutual acquaintance, Ryota Kinoshita (Go Ayano), has dropped by. The volatile wrecker of Tomoko’s first marriage, Ryota disappeared from her life 12 years ago and she is hardly pining for his return. But after Shingo leaves her, sick in the futon, to be with his family over New Year’s, Tomoko gets a call from Ryota — and impulsively invites him over.

Once their affair rekindles with a blazing intensity, the jealous Ryota starts wondering out loud why Tomoko stays with a loser and user like Shingo. And how long, he asks her, does she think she can keep him from finding out about his new rival? Shaken by Ryota’s taunts and rants, which turn violent, she begins to lose her certainty about her relationship with Shingo — or is it just her complacency?

Kumakiri, whose “Nonko 36-sai (Non-ko)” in 2008 also featured a conflicted heroine with an explosive lover, is something of a specialist in human meltdowns that feel like natural disasters. While delivering such a signature scene in “The End of Summer,” he also pays homage to the more restrained cinematic classics of the postwar era by Ozu, Naruse, Gosho and Kurosawa, all cited as influences by Kumakiri. Like these masters, Kumakiri and cameraman Ryuto Kondo can say volumes with a single cut, such as one of Tomoko’s slumped shoulders and lowered head as Shingo walks away yet again — a silent pose that, shot from behind, eloquently gives the lie to her all-is-well front.

Unlike these masters, Kumakiri overdoes the period details, including carefully “antiqued” posters for long-vanished businesses and movies such as 1941’s “How Green Was My Valley” that overshadow the actors and remind us, distractingly, how very long ago it all was.

But affairs of the heart are hardly outdated, with the number of one’s lovers, married or no, still as unfixed as ever.

Fun fact: Jakucho Setouchi was played by Rie Miyazawa in “Onna no Ichidaiki” (“A Woman’s Biogaphy”), a three-part mini-series broadcast on Fuji TV in 2005 about the lives of three famous women.