In this compelling cinematic narrative, three main characters take on their own identities, embodying fictionalized versions of themselves. The story unfolds with Liam Óg "Mo Chara" Ó Hannaidh as he recounts the significant moment of his best friend, Naoishe "Móglai Bap" Ó Cairealláin's birth. This crucial event is marked by Naoishe's mother, Dolores, portrayed by Simone Kirby, who hurriedly takes him to meet his father, Arlo, played by Michael Fassbender, amidst a sacred ceremony. However, their joyous occasion is abruptly disrupted when authorities arrive, implicating Arlo in IRA activities. In the aftermath, Dolores withdraws from the world, confining herself to her home.
As the characters transition into adolescence, Liam and Naoishe find themselves enmeshed in the dangerous world of drug dealing. Concurrently, Arlo vanishes, having staged his own death a decade earlier to elude capture. Despite Arlo's abandonment, Naoishe clings to a profound sense of Irish pride, though it's marred by resentment towards his father for leaving him behind. This emotional complexity deepens when a chaotic party leads to Liam's arrest, forcing Naoishe to escape from the scene.
Later, Liam is taken to a police station, subjected to derogatory remarks from authorities pressuring him to forsake his Irish language for English. In a pivotal moment, Detective Ellis, played by Josie Walker, intervenes by calling in JJ, a teacher focused on interpreting Liam's enigmatic notebook. As JJ examines the contents, he discovers rap verses written in Irish along with acid tabs. With Liam's approval, JJ discreetly takes the notebook, finding inspiration for his musical pursuits.
Throughout his journey with women, Liam often exclaims “Tiocfaidh ár lá!” ("Our day will come!") right before climax, a phrase that intrigues and frustrates many he encounters. Among them, Georgia, a captivating Irish woman, finds herself particularly fascinated by his declaration.
JJ eventually encounters Liam and Naoishe at an arcade, where they are presented with a chance to create music in Irish. Initially hesitant, they eventually embrace the concept, sparking a collaborative process that merges JJ's beats with the boys' rhymes. Adopting the moniker Kneecap, inspired by a notorious torture method from Ireland's tumultuous history, they embark on crafting their first noteworthy track.
As Kneecap's fame grows, the trio lands a gig at a local pub, where an attentive bartender captures their performance on her smartphone. The excitement surrounding their music escalates, and JJ is thrilled to overhear students reciting their lyrics. However, he struggles to maintain a separation between this newfound passion and his relationship with Caitlin, played by Fionnuala Flaherty, who passionately advocates for the Irish Language Act.
Meanwhile, Detective Ellis independently visits JJ and Dolores, warning them about the dangers their music might attract. As Kneecap’s reputation soars, JJ adopts a balaclava and the alias DJ Próvaí to safeguard his privacy. Their pre-performance rituals often involve ketamine, culminating in electrifying performances that captivate their audience. Amidst this frenzy, Naoishe and Liam inadvertently draw the ire of the Radical Republicans Against Drugs (RRAD), who take issue with Kneecap's activities in their territory.
Conflict escalates after Georgia attends one of their shows and is appalled by JJ's provocative antics, leading her to share an unexpected kiss with another man. This confrontation results in Georgia vowing never to see Liam again.
Kneecap's rising star catches the attention of a representative from RnaG radio station, prompting a request for a track for airplay. However, when the trio returns to JJ's garage, they find it reduced to ashes, forcing them to sneak into a school to record new music by night.
Detective Ellis's life takes a shocking twist when she discovers her niece Georgia with Liam in an embarrassing situation, causing Liam to flee in shame. Later, the RRAD confronts Liam and Naoishe, demanding their cut of the profits from Kneecap's performances.
As anticipation builds, JJ and his friends eagerly await their track's radio debut, only to hear it replaced by a rival's explicit song. This sparks outrage among neighborhood mothers, including Dolores, who flood the airwaves with complaints, compelling the station to spotlight Kneecap's music instead. Although this newfound attention brings significant changes, it also leads to JJ losing his teaching job and Caitlin ending their relationship.
Despite these challenges, JJ remains devoted to Kneecap as their momentum climbs. They have a crucial gig where their performance enthralls the audience. To Naoishe’s shock, Arlo makes a surprise appearance, signaling impending chaos. Mid-performance, the boys play a recording of RRAD leader Ellis threatening them, while her henchmen watch from the crowd. A sudden gunshot erupts, inciting panic as everyone scrambles to escape. In the turmoil, Liam is subdued by Ellis, revealing her formidable nature.
After the dust settles, Naoishe finds himself confronted by RRAD members, seemingly doomed. Just as despair seems inevitable, Arlo emerges, seizing Ellis's gun and compelling her men to retreat, expressing pride in Naoishe’s artistic endeavors before allowing him to depart.
As Liam reflects on the aftermath, Kneecap embraces their Irish roots, with Caitlin continuing her advocacy, and Georgia sharpening her Irish language skills. Dolores finally breaks free from her isolation, her voice resonating through the local pub, while Kneecap’s music soars in popularity.
The credits roll with a poignant acknowledgment of the global decline of indigenous languages, underscored by the passage of the Irish Language Act in 2022. Set to the track “H.O.O.D.”, the film concludes with a touching montage of Kneecap's journey, culminating in a heartfelt image of Liam and Naoishe as young boys.
