Full story
Frank Castle thinks he's done. He killed everyone responsible for the deaths of his wife and kids, burned the skull vest, and now works construction under a fake name, barely sleeping. A hacker called Micro reaches out. Micro is David Lieberman, a former NSA analyst who faked his own death and lives in hiding, watching his wife and kids through cameras. He has a video from Afghanistan showing Frank's unit, Cerberus, torturing and killing an Afghan police officer.
The two form a wary partnership. Frank learns the Afghanistan operation was a heroin smuggling scheme run by William Rawlins, a high-ranking CIA officer. The attack that killed Frank's family in the park wasn't random. It was meant to take out Frank, who knew too much. On the other side, Homeland Security agent Dinah Madani is working the same case on her own, because the officer killed in the video was her partner.
Frank has an old ally: Billy Russo, his brother in arms from Cerberus, now the owner of a private military company. Billy is charming, gets involved with Madani, and seems eager to help. But he works for Rawlins and knew everything, including the plan against Frank's family. He kills Madani's partner, Sam Stein. Karen Page crosses paths with Frank again and is one of the few people he trusts.
Meanwhile, Lewis Wilson, a young veteran in the support group run by Frank's friend Curtis Hoyle, falls apart trying to adjust to civilian life. He turns into a bomber terrorizing the city and takes Karen hostage. Frank tries to stop him, and Lewis dies by his own bomb. Soon after, Billy and Rawlins capture Frank and torture him. He escapes, kills Rawlins, and comes out badly wounded.
In the final showdown, at a carousel, Frank fights Billy but chooses not to kill him. He grinds Billy's face into a shattered mirror and leaves him disfigured for the police. Micro goes home to his family. Madani works out a deal, and Frank gets a clean record and a new identity. In the last scene, Frank goes to Curtis's support group and admits he's scared, because he no longer has a war to fight. Billy, in the hospital, is still alive, and that comes back later.