The only thing black and white about “Tabu” is the film. Despite (or maybe because of) its twists and turns, the movie starring Ana Moreira and Ivo Muller works beautifully. (Courtesy of Adopt Films)

There are many strange things about “Tabu,” but the tale it tells is not one of them. The basic contours of the love triangle at the heart of this Portuguese film are familiar: a married woman caught between her husband and another man. But the way filmmaker Miguel Gomes and his co-writer, Mariana Ricardo, lay out the details of that story — or, rather, the way they embed them within another, almost equally fascinating framework — is unexpected.

Most unexpected of all is the way this cliche of forbidden love moves us, with a power that transcends the inherent melodrama. It takes a while to get to the meat of the movie, but it’s well worth the wait.

“Tabu” is basically divided into two almost equal chapters. After a short, fairy-talelike prologue, set roughly 100 years ago, about a man mourning his dead wife, the film jumps to “A Lost Paradise,” in which we’re introduced to Pilar (Teresa Madruga), a middle-aged woman in modern-day Lisbon, and her frail, elderly neighbor Aurora (Laura Soveral). Aurora, a widow with a grown daughter in Canada, is cantankerous to her caregiver, Santa (Isabel Cardoso). In her defense, Aurora appears to be suffering from incipient dementia. One early scene features the old woman sheepishly explaining to Pilar how she has just blown all of her money in a casino, motivated by a dream she had involving a figure who may or may not be a talking monkey.

At this point, your frustration level will be high, understandably. Little of it makes sense, and it’s not clear where exactly Gomes is going with all this. Is “Tabu” about the friendship between women from two different generations, or is it a movie about senility? Two additional subplots — one concerning a young Polish tourist who was supposed to be staying with Pilar, and another concerning Pilar’s friendship with an abstract painter — feel like distractions. Still, they do create the sensation that there is a rich life that exists off-screen, of which we are seeing only bits and pieces.

This is critical, since the film, shot in black-and-white, already looks more like an old movie than the real world. Gomes is signaling two contradictory things: a deep sense of artifice and a deep sense of verisimilitude. Oddly enough, it works.

Eventually, the second chapter kicks in.

Titled “Paradise,” it’s narrated by Ventura (Henrique Espirito Santo), a mysterious man who has been summoned from his nursing home to Aurora’s deathbed by the old woman. What follows in the last hour of “Tabu” unfolds like a silent melodrama, a pantomime enacting the romantic history that Ventura once shared with Aurora, many years ago, in colonial Africa.

Ana Moreira plays the young Aurora, with Carloto Cotta playing the younger version of Ventura. Ivo Muller plays Aurora’s late husband.

Now, it’s difficult to imagine any of this working. The film’s conclusion is essentially a protracted flashback, with no words coming out of the actors’ mouths.

The gimmick is, however, marvelously effective. Maybe that’s because the framing device makes the back story — which appears to be set in the early 1960s — feel less like conventional flashback than like genuine, if flawed, memory. At one point, a rock-and-roll band is shown performing the Ronettes’ 1963 hit “Baby, I Love You” — except it’s the Ramones’ 1980 cover version that we hear coming out of their mouths.

For these and other reasons, “Tabu” is mighty weird. But it’s also surprisingly mesmerizing. Once Ventura starts spinning his yarn to Pilar and Santa, they can’t help but sit, in rapt silence, swept away by the emotion.

You probably will be, too.

Unrated. At the West End Cinema. Contains sex, nudity, brief obscenity and non-graphic gun violence. In Portuguese with English subtitles.
118 minutes.