Kit is a Hong Kong undercover cop who becomes a drug addict to infiltrate a crime syndicate which has been kidnapping people and sending them to Thailand, where the victims are killed and have their organs sold in the black market. The mastermind behind the syndicate, Mr Hung, has a rare heart condition and needs to undergo a heart transplant to prolong his life. Hung's younger brother is the ideal donor for a heart.
When Hung sends his men to kidnap his brother, the situation goes haywire and a gunfight breaks out because the police have been tipped off by Kit about the kidnappers' plan. Hung's brother is injured during the shootout but is rescued by the police. Kit, on the other hand, escapes with the kidnappers but his cover is blown. Hung's men knock him unconscious and send him to a prison in Thailand. The chief warden, Ko Chun, works for Hung and has been keeping the kidnapped victims alive in the prison before they are killed for their organs. Kit is made to serve a life sentence in the prison.
While in prison, Kit attempts to escape twice and fights with the prison guard Chatchai but ends up being subdued every time. Chatchai's daughter has leukaemia and needs to undergo a bone marrow transplant to survive. Chatchai witnesses Ko Chun's illegal activities and brutality but forces himself to remain quiet because he does not want to lose his job. Ironically, the donor who agreed to donate bone marrow to Chatchai's daughter is actually Kit.
Back in Hong Kong, Hung reveals himself to Kit's uncle and supervisor, Wah, who is keeping a close watch on Hung's brother in hospital. Hung threatens Wah to hand over his brother to him in exchange for Kit. Although Wah initially agrees, he changes his mind later and hides Hung's brother in his friend's house. He then leaves his subordinates behind to guard Hung's brother while he tracks down Kit's location in Thailand and travels there.
In Thailand, Wah bribes Kwong, Chatchai's colleague, and enters the prison to find Kit. Shortly after Kit and Wah are reunited, they are discovered by Ko Chun, who captures them and orders Chatchai and Kwong to escort them to the hideout where victims are killed and have their organs removed. At the hideout, Kit and Wah break free from captivity and fight their way out. On the other hand, Chatchai and Kwong change their minds and turn back to help Kit and Wah. Kit, Wah and Kwong manage to escape despite sustaining very serious injuries during the fight; Chatchai is captured by Ko Chun's men, who tie him up and torture him.
In the meantime, back in Hong Kong, Hung's knife-wielding henchman, Ah-zai, kills Wah's colleagues, kidnaps Hung's brother and brings him to a medical centre in Thailand for the heart operation. However, Kit breaks into the medical centre, defeats Ah-zai in a fight, and takes Hung hostage. He then demands that Ko Chun bring Chatchai to him in exchange for Hung. A fight breaks out when Ko Chun and his men show up with Chatchai. Kit and Chatchai team up and defeat all of Ko Chun's men. At the same time, Hung breaks free from his restraints and attacks his brother, but his medical condition deteriorates and he becomes blind as a consequence.
Kit and Chatchai are seemingly no match for Ko Chun after engaging him in a long fight. At one point, Chatchai is knocked out after being stabbed by Ko Chun, who then proceeds to pound Kit against the window until the glass shatters. While in his unconscious state, Chatchai has a vision of his daughter in danger and immediately gets up and knocks Ko Chun out of the window. However, when he realises Kit has also fallen out of the window, he throws out a chain for Kit to hold on to, but Ko Chun grabs the chain instead. Kit then grabs on to Ko Chun's tie and eventually strangles him to death while dangling from his tie. Chatchai stretches out his hand and pulls Kit back to safety. The movie ends with Sa now growing up narrates that she got the transplant and remembers Chatchai hugging his daughter while Kit watches from a distance away.Tony Jaa as Chatchai (阿 猜), a Thai prison guard.
Wu Jing as Chan Chi-kit (陳志杰) "Kit" (杰), a Hong Kong undercover cop.
Simon Yam as Chan Kwok-wah (陳國華) "Wah" (華), Kit's uncle and supervisor.
Zhang Jin as Ko Chun (高 晉), the chief warden of the Thai prison who works for Mr Hung.
Louis Koo (guest star) as Hung Man-kong (洪文剛) or Mr Hung (洪爺), the boss of the crime syndicate.
Ken Lo as Wong Kwong (黃 光) or Kwong (光), Chatchai's colleague.
Jun Kung as Hung Man-biu (洪文彪), Mr Hung's younger brother.
Dominic Lam as Cheung Chun-tung (張振東), Wah's superior in the police force.
Babyjohn Choi as Kwok Chun-yat (郭俊一), Wah's subordinate.
Wilson Tsui as Dai-hau (大口), Wah's subordinate.
Unda Kunteera Yhordchanng as Sa (莎), Chatchai's daughter.
Candy Yuen as Fong Wing-kum (方詠琴), Hung Mun-biu's wife.
Philip Keung as Fan Ging-hung (范勁雄), the boss of a smuggling ring.
Eddie Pang as Hung's henchman
Zhang Chi as Ah-zai (亞 囝), Hung's knife-wielding henchman.
With a budget of US$23 million, filming started on 1 May 2014 and concluded on 6 September 2014.
The first trailer was released on 23 March 2015. The film was released in Hong Kong and China on June 18, 2015.
The film grossed an estimated US$43.36 million in its four-day opening weekend, debuting at No. 2 at the Chinese box office behind Jurassic World and third place worldwide behind Jurassic World and Inside Out.
On The Hollywood Reporter, Elizabeth Kerr called the film "a tight, entertaining action flick". Joe Leydon of Variety called it "a martial-arts noir melodrama that neatly entwines operatic outbursts of emotion with bodacious bouts of butt kicking." Derek Elley of Film Business Asia gave the film a score of 6/10 and called it "a big, splashy mess, with a chaotic script."
A third film installment of the SPL series, Paradox (2017 film) has been released in China and Hong Kong on 17 August, 2017 and 25 August, 2017 respectively.