In Kirill Serebrennikov's adaptation of Aleksei Sal΄nikov's 2018 novel, the Petrov family's bout of flu produces a hallucinatory merging of everyday life, memory, and fantasy. The film invokes paganism and Greek and Roman mythology, blending Russia with a kind of Hades/underworld. At one point, the Petrovs’ son asks, “Is it true that when a murderer dies, the ghosts of everyone he killed gather at his bed?” The film correlates their illness with the social ills plaguing Russian society, connecting the personal and familial to the national. Side characters utter xenophobic complaints about immigrants or call for changes to the political system, giving voice to the socio-political dynamics of Russian society. Outbursts of shockingly graphic violence punctuate the film: executions, fights staged as action-film sequences, extreme violence against children, brutal stabbings, and an assisted suicide with a gun. Alongside these dramatic interludes, the film explores family dynamics, specifically examining how the two parents care for their sick son and comparing the father's and son's childhoods.

The film focuses primarily on Petrov senior (Semen Serzin), a car mechanic in the process of divorcing his wife (Chulpan Khamatova). They still live together, along with their preteen son (Vladislav Semiletkov). The older Petrov's perception of reality deteriorates under the influence of flu and alcohol. He goes on a bender with his loquacious friend Igor, who picks up Petrov in a neon-lit hearse. Vodka and a high fever trigger childhood flashbacks, showing a first-person view of Petrov's own parents and a New Year's Eve party. He also remembers helping a friend kill himself. Petrov's wife, a librarian-cum-vigilante, dispenses violent retribution to a man who attacks a poetry reading and then to a co-worker's wife-beater husband. She also has violent fantasies about slitting her own son's throat. Serebrennikov intertwines these dark fantasy sequences with episodes focusing on Petrov and Petrova's care for their son. In the finale, the Petrov plot cedes way entirely to the story of Marina (Iuliia Peresil΄d), a young worker from rural Russia in the late 1970s who performed the role of Snegurochka at the New Year's party Petrov remembers from his childhood. The film marks these different narrative strands with distinct formal and visual elements: Petrov and Petrova's present-day episodes use 2.35:1 Cinemascope aspect ratio and dim lighting; Petrov's childhood memories invoke the home movie 4:3 ratio and use natural light; and Marina's episode returns to the anamorphic 2.35:1 but in black-and-white.

Petrov's Flu reflexively engages with the process of artistic production, contrasting literary delusions of grandeur, local poetry gatherings, and individual endeavors such as Petrov's comic book illustrations. Formally, the film amalgamates fantasy, memory, and contemporary reality through the digital manipulation of shots, creating a surreal continuity between the waking world and the characters’ thoughts and urges. The expansive opening shot emphatically establishes this continuity: Petrov rides a packed bus in a feverish daze, and after several passengers address him, imagines being pulled off the bus to participate in the execution of a group of formally clad men and women. Similarly fantastical, illogical continuities appear through the reoccurrence of varied objects, details, and background characters. An accordion player inexplicably appears throughout the film. Petrova's library supervisor knits the same sweater as the one Marina wears, impossibly, in the 1970s. The dentures of an aging misogynist reappears several times in the film, moving autonomously and later singing the soundtrack's most recognizable song.

Against impossibly fraught aesthetic, socio-political, and historical frameworks, the dreamlike continuities that tie together the pieces of Petrov's Flu define Serebrennikov's essential strategy: he inundates both realistic and hallucinatory scenes of the film with details suggesting but never closing tantalizing possibilities of interpretation.