Hsing, a local gangster who operates illegal migrant work as caregivers in rural Taiwan, fails to pay his workers for two months, stirring up dissatisfaction.

Hsing and Oom then drive them to a karaoke bar in town, revealing that they are actually selling them to a human trafficking organization led by a gang boss named Brother Te.

Hsing then decides to recruit new migrants from Te's trafficking operation, forcing Oom to participate in the abduction to replenish their caregiving staff.

In 2017, director-screenwriter Chiang Wei-liang envisioned creating a film centered on migrant caregivers in Taiwan, motivated by his uncle's experience of being bedridden due to a stroke caused by language barriers and a lack of cooperation between his family and their Myanmar domestic helper.

[6] Chiang, a Singaporean expat based in Taiwan, had previously shown interest in exploring the situation of the South Asian diaspora in Taiwan, producing the short film Nyi Ma Lay (2017) and the VR film Only the Mountain Remains (2018) on this topic, which he described as driven by "a sense of social responsibility" as a filmmaker due to his own similar background.

[7] The plot was also inspired by Chiang's personal experiences while taking care of his mother in Singapore,[2] as well as his societal observations and encounters with other migrant caregivers.

[8] In June 2020, the project was awarded funding by the Centre national du cinéma et de l'image animée, receiving a development grant of 5,000 euros.

[8] Rungkumjad spent five months in Taiwan preparing for and shooting the film, which included learning Mandarin and communicating with illegal migrant workers.

[8] Cinematographer Michaël Capron was invited by French co-producer Marie Dubas to lead the shoot while he was filming in Bogotá, and he immediately accepted the offer after reading the script, drawn to its writing and subject matter.

[15] To create a sense of imprisonment for the lead character Oom, the production crew used backgrounds with blank walls and small windows for interior shots; while for exterior shots, direct sunlight was also kept to a minimum, prompting the crew to choose sheer mountain ranges in eastern Taiwan for their mood-suitable weather, with scenes filmed at altitudes of 2,000 to 3,000 meters and Capron described the shooting process as "difficult".

[27] Peter Bradshaw of The Guardian gave Mongrel 4/5 stars and described it as a "sombre, sober movie made with impressive artistry", highlighting the "Zen state of suffering and sadness" that poignantly captures the despair and exploitation faced by illegal migrant workers, particularly through the lead character Oom.

[28] John Berra of Screen International also recognized director Chiang Wei-liang's thematic focus on illegal migrant workers, describing the film as a poignant yet powerful character study that effectively explores their challenges through an unflinching presentation marked by moral ambiguities, enhanced by Michaël Capron's careful use of cramped interiors and night-time exteriors, as well as Dounia Sichov's "docudrama feel" editing.

[3] Stephanie Bunbury of Deadline Hollywood called the film "an absolutely brilliant piece of filmmaking", praising director Chiang Wei-liang's masterful control of pacing and visuals that evoke a sense of sci-fi dystopia, along with the rare decision to end the film with a "brief flash of optimism" amidst its harrowing subject matter.

[4] Namrata Joshi of Cinema Express described the film as "sombre, sobering, and ambiguous", noting its "rare grace amid utter despair" and highlighting challenging yet deeply humane exploration of migrant workers and emphasized the moral dilemmas reflected in the characters' struggles.

[29] Josh Slater-Williams of IndieWire gave the film a B and offered a rather critical review, noting its similarities to Sofia Coppola's Lost in Translation (2003) and emphasizing that the visuals, editing, and pacing made it challenging for audiences to fully absorb its themes, although the messages about the struggles in rural Taiwan and the Southeast Asian diaspora still resonate amid the film's grim and impactful techniques.