In the Ironside Brewery's palatial boardroom, Ironside hectors his executives into increasing production at the expense of quality. A lone voice speaks up against him but is ruthlessly quashed. Ironside wants domination of the British beer market. Only one small brewery, Greenleaf, still holds out.

Ironside visits his son, the firm's advertising executive. Roles are reversed as the son barks orders at his father. He has arranged for his father to visit Greenleaf and offer to buy it outright.

Ironside arrives in a vast limousine at the rural Greenleaf Brewery. Nothing has changed here for a hundred years. Inside the brewery, master brewer Matt Boyle tastes his new brew and deftly separates two labourers, Albert and Geordie, who customarily fight with the slightest provocation. A friendly employee offers Ironside's chauffeur a drink.

Margaret, Greenleaf's daughter, drives recklessly fast, straight into the back of Ironside's car. Walking into the office, where Ironside is bullying Greenleaf into selling his pubs, she smartly refuses the offer and shows him the door. Outside, all is chaos as Geordie and Albert manage to wreck the car. Ironside, finding his car in a dreadful state, is forced to drive himself as the Chauffeur is drunk on Greenleaf's brew.

Matt asks Margaret to marry him, but she gently brushes him off. John Ironside berates his father for being bested by a girl and sets out to wine and dine Greenleaf's daughter, under the impression she is a harridan. At a Greenleaf inn, Margaret drives into the back of John's car and he spanks her like a child. Unaware of each other's identity, they wait in the lobby and make up, clearly attracted to one another. When he learns who she is, John decides to infiltrate the Greenleaf business as their advertising manager, planning to spend all the money on an ad campaign, concealing the costs and driving them into bankruptcy. He meets Matt and, under the influence, persuades him to give him the job. Matt tells John about his grandfather's secret beer recipes, including one containing 'all the sorrows of Ireland' which would make the drinker weep.

Next morning, John begins an ambitious and expensive ad campaign. Margaret presides over rehearsals for the concert to celebrate Greenleaf's150th anniversary, to consist of a monologue and an old-fashioned part song. As John reports back on the success of his stratagem, Ironside Senior reads Mein Kampf. John is called into Greenleaf's office to be told there is a small increase in profit. The sharp-eyed accountant asks about the advertising costs and when Greenleaf congratulates the boy, cynically remarks that he would indeed be well worth watching.

Unnerved by this directness, John has a sudden attack of conscience, and realises that he doesn't want to ruin Greenleaf's. He tells his father he will sell his shares and pay the advertising costs himself. Out for a hair-raising drive with Margaret, during which she lands them in a ditch, he proposes. Meanwhile, Matt asks Greenleaf to speak to Margaret about a marriage so he can announce it at the concert. When Margaret tells her father she is engaged, he draws the wrong conclusion.

At the concert, the vocal club performs The Two Obadiahs, followed by an impromptu jazzing up of Albert and Geordie's song. Matt performs The Retreat from Moscow with disastrous interventions by the pair. Greenleaf announces that Margaret is engaged to Matt. In the ensuing confusion, Ironside Sr. arrives. He states his intention of putting them out of business.

Thugs hired by Ironside stage brawls in Greenleaf's pubs. Greenleaf and his accountant discuss the affect it has on the business; the accountant remarks that they must fight back.

John return to Greenleaf's to help, but is rebuffed by Margaret. Matt goes on a bender and, drunk, visits Ironside and gives away his Greenleaf shares, giving Ironside a controlling interest. John, desperate to redeem himself in Margaret's eyes, hatches a plan to use Matt's weeping beer to spike the Ironsides brew, putting them out of business. He then forces his father into a merger, in which Ironside's pubs will sell Greenleaf's beer. Although Margaret and her father disapprove of John's methods, the merger and wedding take place.