El Vaivén centers on a young woman who works in a fish market in Mexico City. While wrapping a fish in plastic, she becomes captivated by her own reflection in the animal's eyes, sparking a moment of empathy that quickly spirals into a suffocating sense of claustrophobia.

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The oppressive atmosphere of the densely populated city intensifies her distress. The streets are filled with death imagery as the annual protests commemorating the 1968 student massacre merge with the outcry over the 2014 kidnappings of the Ayotzinapa students, blending with the Day of the Dead celebrations. People with skeleton face paint roam the city, while others protest the violence that plagues the nation. Piles of dead animals in the market evoke the mass graves frequently reported in the news, further fueling her unease.

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As her anxiety mounts, the young woman begins to transform into a fish, suffocating in a city suspended between life and death. Disgusted by her job and her surroundings, she decides to flee the city. She takes a bus to the ocean in Veracruz, where she completes her transformation and swims away, seemingly free at last.

 

However, her journey comes full circle when she is caught by a fishing boat, transported back to the same market in Mexico City, and wrapped in plastic by none other than herself. The film concludes where it began, revealing that the fish that initially evoked her empathy was, in fact, her future self, caught in an endless cycle of violence and entrapment.​​​​