Following the death of his wife Helen from an unknown illness, John Wick receives a puppy named Daisy as a final gift from her to help cope with her loss. Days after her funeral, John is confronted by a group of gangsters led by Iosef Tarasof at a gas station who try to intimidate him to sell his most prized possession, a 1969 Boss 429 Mustang. The same gangsters later break into John's house, where they brutally beat him before murdering his puppy and stealing his car. Iosef then takes the car to a chop shop, but Aurelio, the shop owner, refuses service as he recognizes the vehicle. He later tells John that Iosef is the son of crime boss Viggo.
Upon learning of Iosef's actions and his target, Viggo later beats his son for incurring John's wrath. He tells him that John was once a hitman on his payroll under the name "Baba Yaga", the most feared in all of the criminal underworld, a ruthless man of focus, commitment, and sheer will. When John fell in love with Helen, Viggo gave him a seemingly "impossible task" to win his freedom... which he accomplished.
John recovers his weapons from his past career, as well as gold coins used as currency for his services. He refuses to make amends with Viggo for his son's actions and kills the hit squad that break into his home. Viggo then puts a $2 million bounty on John's head and hires his old friend Marcus to take him out. John takes refuge in the Continental, where business cannot be conducted and serves as neutral ground for assassins. The hotel's owner and an old friend of John's, Winston, tells him that Iosef is hiding in the Red Circle nightclub. John infiltrates the club and confronts Iosef, but is forced to flee after a fight with Kirill, Viggo's chief henchman.
While resting back at the Continental, John is attacked by female assassin Ms. Perkins, who had sneaked into his room. Marcus fires a warning shot to alert John, and John subdues her. She then reveals that Viggo doubled the bounty to kill him at the hotel. She also says that Viggo has a high-value stash concealed in a church. John then sends another hitman named Harry to secure Perkins, but she escapes and kills Harry in the process.
John raids the church and destroys Viggo's stash, consisting of a cache of cash and blackmail material. When Viggo arrives, John attacks him and his men but is captured. While in captivity, John tells Viggo that he will not stop until he kills Iosef since the puppy had given him hope after Helen died, and that Iosef had robbed him of that hope. Marcus once again intervenes and saves John, allowing him to kill Kirill and force Viggo to give him his son's whereabouts. John soon arrives at the safehouse Iosef is hiding at and kills him, avenging his puppy. Marcus then encourages John to return to his normal lifestyle, but Perkins, witnessing the meeting, reveals Marcus's duplicity to Viggo, who has him tortured and killed.
As a result of disobeying the Continental rules, Perkins is executed by several assassins under Winston's behest, who tells John that Viggo is attempting to escape in a helicopter. Racing to the New York Harbor, John kills Viggo's remaining henchmen before confronting Viggo himself. After a brief fight, John fatally stabs Viggo and says his goodbyes before leaving him for dead. As John begins bleeding out from his wounds, he watches a video of Helen and treats himself at a vetinary clinic, before picking up a pitbull puppy and heading home.
