Amores perros

The film is constructed from three distinct stories linked by a car accident that brings the characters briefly together.

Octavio y Susana
The first segment stars Gael GarcÃ­a Bernal and Vanessa Bauche as the title characters. Susana is Octavio's sister-in-law; however, Octavio is in love with her and does not like the way his brother, Ramiro, treats her. Octavio tries to persuade her to run away with him to get out from under Ramiro's abuse. Jarocho, a local thug, is happy after winning in a dog fight, so he decides to let his dog loose on some strays, but is stopped when a vagrant pulls out a machete from his junk cart; eventually, Jarocho sees Octavio's rottweiler, Cofi, wandering the streets, and lets his dog loose on it, but Cofi kills Jarocho's dog. Aware of this by his friend Jorge and needing to make money so that he and Susana can escape and start a life of their own, Octavio decides to become involved in the business of dog fighting, organized by Mauricio, owner of El Sabadaba, where the fights take place. Every time there is a fight, Jarocho enters a new dog, and everytime, Cofi kills it. After more than fifteen fights, Octavio makes enough money to run away with Susana, even paying Mauricio to get Ramiro beaten up. In revenge, Ramiro steals the money and leaves with his wife. Struggling financially, Octavio accepts a challenge by Jarocho to participate in a private fight, with no outside bets. Octavio's dog wins, but Jarocho shoots Cofi. In revenge, Octavio stabs Jarocho in the stomach. Pursued by Jarocho's thugs, Octavio finds himself in a car chase with Jorge, and the wounded dog. A collision follows; Jorge dies and Octavio is badly injured.

Daniel y Valeria
The next segment stars Ãlvaro Guerrero and Goya Toledo. Daniel is a successful magazine publisher who leaves his family to live with his lover, Spanish supermodel Valeria. On the day they move together, Valeria's leg is severely broken in the accident with Octavio's car and she may be unable to continue working as a model. Valeria is using a wheelchair while she recuperates in the apartment she shares with Daniel. One day, her dog Richie disappears under a broken floorboard and stays there for days. The missing dog triggers serious tension for the couple, causing numerous fights which lead to doubts about their relationship on both sides. Trying to help the dog, Valeria re-injures her leg; Daniel finds her hours later, resulting in severe internal bleeding and eventually gangrene. Her doctor is forced to amputate the leg, removing any chance she might have had at returning to her modeling career. Once her leg is gone, Valeria realizes that her life is most likely ruined, since her sense of purpose, modeling, has been taken from her. Later, we see Daniel calling his estranged wife to hear her voice and then hanging up without speaking, suggesting either that he regrets his disloyalty to her, that he realizes that his attraction to Valeria was based only on her now lost beauty, or perhaps both.

El Chivo y Maru
The final segment stars Emilio EchevarrÃ­a and Lourdes EchevarrÃ­a. The story follows the bedraggled vagrant that appears several times in the previous segments, pushing a junk cart accompanied by several mongrel dogs for whom he cares. Though he appears to live in perpetual squalor in an abandoned warehouse, he is in fact a professional hitman, known as El Chivo (The Goat). As revealed by Leonardo (a corrupt police commander), El Chivo is a former private school teacher who, embracing idealistic notions and goals, became involved in guerrilla movements, committing several terrorist attacks that landed him in prison for 20 years. When he got out, Leonardo started getting him jobs as a hitman. At times throughout the story, El Chivo tries to make contact with his daughter, Maru, whom he abandoned when she was a two-year-old child when he began his guerrilla involvement. Following El Chivo's wishes, Maru's mother told Maru that her father had died, instead of telling Maru the truth about her father's abandonment and prison sentence.
As the story progress, El Chivo is hired by a businessman to kill his partner. El Chivo is about to make the kill when the film's central car crash interrupts him. During the chaos at the crash scene, El Chivo steals Octavio's money and takes his wounded dog Cofi to his home to nurture it. While El Chivo is away from the warehouse one day, following his target, Cofi, due to his proclivity for dog fighting, kills all of the other dogs in El Chivo's house. El Chivo is intensely upset and prepares to kill Cofi, but El Chivo forgives Cofi, figuring that Cofi does not know any better, and seeing Cofi's violence as a reflection of his own life as a hitman.
Meanwhile, Ramiro and an accomplice are attempting to rob a bank when Ramiro is shot and killed by Leonardo's plain-clothes bodyguard. At the funeral, Octavio, seriously injured from the car accident, sees Susana for the first time since she and Ramiro fled with his money. Despite having been wronged, Octavio tries again to get Susana to run away with him, but she becomes angry with the fact that Octavio is willing to run away with her after she has just lost somebody she loves; he tells her that he is still going on with his plan to leave for Ciudad JuÃ¡rez. A few days later, Octavio is shown waiting at the bus station for Susana. She never shows, and Octavio does not get onto the bus.
Still grieving for his beloved dogs, El Chivo captures his intended victim, and, after learning that the victim is the client's half-brother, also captures his client. After shaving his unkempt beard and grooming his hair he leaves both men alive and chained to separate walls with a pistol within reach between them, their fate left undetermined. El Chivo then breaks into his daughter's house while she is away and leaves her a large bundle of money along with a message on her answering machine explaining what happened to him and why the family was split. Just before El Chivo was to tell his daughter Maru that he loves her, the answering machine stops recording. El Chivo then goes to an autoshop, where he sells the client's SUV. The mechanic asks him the dog's name, and El Chivo calls him "Negro" (Spanish for "Black"). After El Chivo receives the money for the car, Chivo and Negro walk away, disappearing into the horizon.

Question: What links the three stories in the film?
Answer: a car accident