It’s a 40-minute flight, flying east over the rainforest and the tangled channels of the mighty river. I gaze out at Parintins as the plane makes its final approach.

I can see dozens of boats moored up to three abreast on the riverfront; they serve as makeshift hotels for the duration. Two giant flags — one blue, one red — languidly flap at opposite ends of the town center. Midway between the rival camps lies the Bumbodromo, a purpose-built 35,000-seat stadium that serves as the venue for this annual competitive carnival, the Boi Bumbá.

Everyone arriving in Parintins during Boi Bumbá chooses his garb deliberately. I am wearing a neutral green polo shirt. Most of my fellow passengers are dressed in either blue or red, proclaiming their allegiance.

The division by color is so acute that famous brands are forced to take radical action to avoid accusations of favoritism. Coca-Cola, for example, is sold in cans that are half-red, half-blue. The Parintins branches of Banco Bradesco have blue logos in addition to the usual red. Airlines, vehicle manufacturers and supermarkets have similarly adopted the two-tone solution for local advertising.

At 9 p.m., I am in the bleachers of the packed Bumbodromo. Each team is allotted two and a half hours to present its show, complete with dancers, musicians, drummers, singers, gaudy costumes and ornate floats that fit together to make immense, moving stage sets. The red team — known as Boi Garantido, the “Guaranteed Ox” — goes first. Its supporters, occupying half the stadium, are an integral part of the performance, waving props, chanting and singing on cue. I’m on the blue side — Boi Caprichoso, the “Capricious Ox” — where the lights have been dimmed and all the spectators sit impassively, studiously blind to the incredible spectacle being played out on the field.

Both sides must tell the same folk story, about a farm worker and his pregnant wife’s craving for ox tongue. Each performance must include the same characters and narrative elements. High in the stands, a panel of judges assesses the technical and creative merits of the rival shows. They will determine the ultimate winner.

At midnight, the red side of the stadium dims, and the spotlights light up the blue. The excitement is infectious. I am bombarded with noise and dazzled by the succession of floats representing aspects of Amazonian life. At the height of the show, a huge model of a blue and gold macaw flaps to the blue stands. A female dancer climbs onto it, then rides it as it flies out over the field. The blue fans erupt. The reds sit with their arms folded.

The following day, I leave the festival behind, returning to Manaus. From there, I head off to experience the Amazon firsthand. Even in the remote backwaters, people are discussing Boi Bumbá, tensely awaiting news of the judges’ verdict. Everyone is committed to one side or the other (many families are divided between the two colors).

Finally, the astonishing result is announced. The reds are victors by a fraction of a point. The fragility of the margin compounds the rivalry and will only serve to intensify the competition next year.

My indifference has been swept away by my unforgettable night in the Bumbodromo. Go Caprichos!