The film is based on the real-life 2010 Heung Wo Street murder, in which a mentally unstable son killed his mother and sister, leaving his father as the sole survivor.

Philip Yung was initially attached to the film adaptation of the murder case as the screenwriter in 2011 and interviewed the surviving father Kan Fuk-kui while researching the screenplay, before the project was stalled in development.

The film adapts a nonlinear narrative:[1][2] Nin returns to his restaurant in a morning, only to find his colleagues and neighbours looking at him strangely, revealing that his son Ming has murdered his wife Yin and daughter Grace, and turned himself in to the police the night before.

He attends his son's trial, where Ming confesses and reveals that he committed the murders because he heard a voice telling him he has a mission to eliminate a portion of the population and save the world.

One day, Grace brings home a stray cat, naming it Carnation, and insists on keeping it despite Ming's objections about limiting the animal's freedom.

[13] Kan Ka-leung was tried and convicted for manslaughter in 2012,[16] with his psychological assessment diagnosed him with schizophrenia and revealed that he suffered from auditory hallucinations during the murder, hearing voices that told him the world was overpopulated, wasting resources and harming the environment, and that he had a mission to eliminate part of the population.

[18][19] To research for the screenplay, he personally approached Kan Fuk-kui with the help of then-district councilor Leung Yiu-chung and solicitor Mary Jean Reimer and listened to him share his story.

[20][21] Kan supported the idea of adapting his experiences for the big screen,[22][23] aiming to share his rare insights and draw attention to people with mental illness.

[19] Unlike the typical approach of focusing on the murder as a thriller, Yung aimed to tell the story from the father's perspective and raise awareness of mental illness.

[18][11] He also employed a nonlinear narrative with disjointed flashbacks while writing the screenplay, explaining that this approach was intended to replicate human memories, which "do not function linearly and can be fragmented".

[11] Although Lau was initially hesitant to accept the role due to the character's complexity, he read the script multiple times and ultimately found himself drawn to it, agreeing to participate in the project.

[21][32] Yeung Wai Lun, the lead actor from The Sparring Partner (2022) produced by Yung, returned to star in the film and provided advice to So on portraying a murderer.

[22] After producing The Sparring Partner, Yung found himself in debt and had to make a low-budget Chinese romance film, Penny Pinchers (2024), to repay it while on holiday in 2022 and further postponed production.

[25] Although the murder took place in Tsuen Wan, Philip Yung opposed the idea of shooting on location in the district to avoid bringing back tragic memories for local residents, except for the scene where Ming fled after the murder was filmed on location at Tsuen Wan Riviera Park [zh] in accordance with the real-life events.

[49] A member of the boy group Mirror was reported to have a cameo appearance in November,[50] and Philip Yung revealed that it was Edan Lui in the same month.

[53][54] Keith Ho of HK01 described the song choice as "a masterstroke", noting that the lyrics captured Nin's inner struggles, his beautiful memories with his wife, and his motivation to reconcile with his son;[55] while Cyrus Lamprecht, writing for Hong Kong Economic Times, also acknowledged the choice of the theme song, noting that it was also used as an interlude in Ann Hui's The Story of Woo Viet (1981), and compared the similar themes of the two films, both conveying the message of the protagonists letting go of their obsessions and freeing themselves from their tragedies.

[71] However, audience reactions were mixed,[72] with Sing Tao Daily highlighting criticisms regarding whether the film inflicted secondary victimisation on the surviving father from the Heung Wo Street Murder.

[73] Richard Kuipers of Variety praised Papa for its "low-key presentation that creates high emotional impact", emphasizing its "great depth and complexity" and highlighting Sean Lau's "terrific performance" and Philip Yung's skillful direction, which effectively balances emotional depth with an exploration of love and loss in the aftermath of tragedy through a nonlinear narrative.

[4] Jordan Mintzer of The Hollywood Reporter described the film as "well-crafted and intriguing", exploring the aftermath of a tragic crime through the eyes of a father grappling with grief and seeking understanding, ultimately offering a complex reflection on redemption interspersed with moments of dramatic frustration.

[74] Edmund Lee of South China Morning Post gave the film 4/5 stars, summarizing it as a "psychologically complex tale of unconditional family love, beautifully narrated around an impossibly macabre twist", while praising the 4:3 aspect ratio and fragmented flashbacks that create an "emotionally genuine slice-of-life drama" and highlighting the uniformly excellent ensemble, especially Sean Lau's poignant performance as the conflicted father, which provides a well-crafted exploration of grief and love despite its true crime roots.

[75] Amy Mullins of China Daily acknowledged the film's "intimate, gritty" storytelling and its innovative use of time and visual style to convey the protagonist's emotional turmoil, suggesting that it offers a soulful exploration of trauma without judging its characters.

[76] Calvin Choi, writing for Hong Kong Economic Times, expressed his admiration for the film, commending its focus on the emotional aftermath of a tragic crime rather than the crime itself, and praises the film's nuanced exploration of the father's grief and memories, highlighting the complexity of relationships, particularly the father-son bond, while acknowledging the emotional difficulty it evokes.