Agnes Brown (Brendan O'Carroll) is an independent market trader, selling fruit and vegetables on Dublin's Moore Street market. It has been under attack from P.R. Irwin (Dermot Crowley), a TD (PRIC) who is in an arrangement with a ruthless Russian businessman who wants to put all the market stalls out of business and open a shopping centre on the site. Her stall is the next to be targeted, being sent a bill for unpaid tax left by her grandmother, and a man (working for Irwin) appears offering to buy her stall and make the bill disappear. Agnes nearly accepts, but Winnie (Eilish O'Carroll) reveals this news to the locals, forcing Agnes into defending her stall from the developers while they look for ideas on how to raise the money. Agnes' friend Philomena Nine Warts informs her that her grandmother, Mary Moccasin, was next to Agnes' grandmother at the tax office when she paid the bill and therefore no money is owed. Unfortunately Philomena's grandmother is hit by a bus on the way to the courtroom before she can testify.

Agnes' court case attracts a lot of attention from the media, portraying her as the greatest mother in Ireland. This leads her to go to confession, where she admits (unknowingly also to a Russian mobster) that she briefly put her children in care when her husband died, but continued to claim the child support money. This is used against her in the witness box during questioning by Irwin in court (Irwin being the opposing Barrister), and she runs out in shame. Eventually being found by the river by her daughter Cathy (Jennifer Gibney), she admits all in a tearful moment on the Ha'penny Bridge, telling her how she told the nuns that she thought she could look after two of the six children, but when asked to pick she was unable to.

Meanwhile, Agnes' son Dermot (Paddy Houlihan) and his lifelong criminal best friend Buster Brady (Danny O'Carroll) try to get the receipt. After failing to break into the restricted area of the NRS they recruit a troop of blind trainee ninjas, led by Mr. Wang (also played by Brendan O'Carroll). The Russians have already found and destroyed the original receipt, but Buster and Dermot learn the receptionist that took the payment was blind, so that there exists a braille version of the receipt. They find it and let Agnes know, telling her Tourettes-suffering barrister (Robert Bathurst) to stall the case. After navigating air ducts out of the NRS, Agnes, Buster and Dermot are chased by the mobsters and the Garda, jumping in the River Liffey. Agnes separates from the pair and returns with the "receipt" but it turns out Buster accidentally gave her a betting slip instead. At this point Cathy stands up and gives a speech on how special Moore Street and its market is, and her intention to run her mother's stall when her time comes, to Agnes' joy. After their pursuit continues in a Nissan Navara and finally a dash on a stolen horse, Buster and Dermot deliver the receipt to the court room just in time to have the case against Agnes dropped. They all celebrate by dancing on the steps of the courtroom.