In this prequel to 'The Texas Chainsaw Massacre: the Beginning', the film opens in rural Texas in the year 1955 at the Sawyer household, a birthday party is being held by the family matriarch Verna Sawyer (Lili Taylor) for the youngest member of the family; Jedidiah. As his present and to induct him into the sadistic family rituals, he's presented with a chainsaw and forced to torture a man who they've accused of attempting to steal one of their pigs. Jed refuses, visibly disturbed, and the thief is killed by Grandpa.
A few months later a young couple, Betty Hartman and Ted Hardesty, are driving through the family's territory when they come across a seemingly wounded Jed. Betty follows him to a dilapidated barn where she's promptly killed by the family. A few hours later, Betty's father Sheriff Hartman (Stephen Dorff) arrives to find her dead. As Verna arrives to protect her sons, Hartman quickly takes Jed into his custody as revenge, sending him to a mental asylum for disturbed youths.
The film skips forward 10 years later to 1965 where a new nurse is starting at the mental health facility. The new nurse is named Lizzy (Vanessa Grasse) and she quickly forms a bond with patients Bud and Jackson (Sam Strike) who is oddly charming and respectful. Verna arrives later that night with an injunction in attempt to have the teenage Jed freed but is rejected by the director of the facility. She breaks in, causing a riot during which many of the nurses and patients are killed. Lizzy is saved by Jackson who gets her outside to safety where the two are promptly taken hostage by two crazed, over-sexed escapees named Ike (James Bloor) and Clarice. As they leave, they come across a bloodied Bud, who has just killed the hospital administrator and they decide to take him with them.
The group arrive at a rest stop diner shortly after changing clothes. Following an altercation with waitress Tammy (Nicole Andrews), Ike and Clarice quickly begin to brutally kill the patrons including a mortally wounded Tammy, in the scuffle Bud is shot and the group flee. Sheriff Hartman arrives on the scene with Deputy Sorrel (Finn Jones) and discovers that one of the group is in fact the teenage Jed and quickly sets off after them.
That night the escapees take refuge in an abandoned mobile home where Clarice and Ike make love revealing her scarred body and involve the corpse of the mobile home owner in the debauchery. While everyone is asleep, Lizzy tries to escape but is stopped. Ike continually insults Bud and is later killed by him. The next morning Clarice notices Ike is missing and heads off to find him. Jackson and Lizzy find Bud sleeping on Ike's corpse and quickly take him away.
Clarice is apprehended by Hartman. She is hostile and refuses to co-operate and insulting both him and his dead daughter. Hartman then shoots Clarice in the head as the others watch from a distance. Jackson and Bud continue on with Lizzy in tow. As a Deputy passes, Lizzy screams out only to inadvertently get Bud killed. Bud's death sends Jackson into a rage, he kills the Deputy and has Lizzy drive them in the cop car to safety.
During their drive the two are pursued by a now insane and deranged Hartman who opens fire on the two, seemingly killing Jackson and causing Lizzy to careen off the side of the road.
Later that night, Lizzy, finding herself handcuffed in a cop car, answers a dispatch call alerting another officer, Sorrel, where she is. Sorrel visits Verna and reveals to her that Hartman has her son, Jed, at the old barn where Betty was killed and Hartman had Jed taken away. Verna has Sorrel killed and fed to the pigs and heads to the barn to save her son. In the barn, Jackson is revealed to be alive and Hartman reveals to a horrified hostage, Lizzy that Jackson is in fact Jed. Verna arrives and has Hartman incapacitated.
Back at the Sawyer house, Verna stitches up the large wound in Jed's face and applies a muzzle to hold his face together. She then leads Jed into a room where his siblings are holding Lizzy and Hartman captive. She gifts Jed his original chainsaw and Lizzy watches on in horror as Jed bisects Hartman, killing him.
As the family celebrates, Lizzy escapes fleeing into the woods and the Sawyer family give chase. She reaches a main road just as Jed catches up to her, but his chainsaw begins to malfunction. She hesitates until he gets it started and proceeds to chase her down a long road slowly gaining ground. Lizzy almost escapes but is caught in a bear trap. She attempts to appeal to Jed's good nature, having seen the good within him, as Verna tries to encourage him to kill Lizzy to protect their family. Jed begins to soften as Lizzy pleads with him, insisting that he is not a killer until she insults Verna, calling her crazy at which point he brutally decapitates her.
The next morning Verna is seen burning evidence of the night's events as the elder boys feed ground up remains to the pigs. Down in a basement Jed is seen creating his very first mask from Lizzy's face which he has sewn together to become Leatherface.