Two Irish brothers become political and romantic rivals.

In 1854, Irish immigrant Patrick O'Leary, traveling by covered wagon with his wife and three young sons to Chicago, is killed when he races a train and the wagon crashes down an embankment. Before he dies he tells his sons Jack, Dion and Bob to build and grow with Chicago, which he predicts will one day be the hub of the country. In the city, Patrick's wife Molly opens a successful French laundry. In 1867, her cow Daisy kicks Bob into an embrace with Gretchen, a servant girl, and they soon marry. Dion, a gambler, falls in love at first sight with Belle Fawcett, the newly arrived singer at The Hub, a saloon in the disreputable part of town known as The Patch. After several unsuccessful attempts to approach Belle, Dion appears in her dressing room, wrestles her to the ground, kisses her ear and succeeds in interesting her in his proposition that together they open up a saloon to rival The Hub. Their saloon, The Senate, proves to be very popular, and Gil Warren, the owner of The Hub, offers to close down and gives Dion $10,000 for his support in his campaign for mayor. Dion, however, secretly organizes a committee to call upon his brother Jack, an idealistic lawyer, to run as a reform candidate. Jack accepts but warns Dion that if he wins, he will wipe out corruption in The Patch. To prevent Warren from winning, Dion arranges a brawl on election day so that Warren's repeat voters are locked up, and he forces Warren's unscrupulous poll watchers and judges to leave town for the day. Jack is elected, and he immediately declares war on The Patch, planning to have the area, which he calls a fire trap, condemned and torn down so that it can be rebuilt with steel. When Jack convinces Belle, who is now engaged to Dion despite Molly's spirited objections to her occupation, to help, Dion angrily reveals that he got Jack elected and tells Belle that if she is with the reformers, she will not be seeing much of him. The day before Belle is to testify against Dion, he proposes to her and convinces Jack to marry them that night. After the wedding, he states that now Belle cannot testify against her husband, whereupon Jack socks Dion and promises to ruin him. Meanwhile, at the O'Leary house, when Molly learns from Gretchen about the fight, she leaves Daisy nursing a heifer. Daisy responds to a sharp tug from the heifer by kicking over a lantern, and a fire starts in the barn. Because it has not rained in three months, the fire spreads quickly throughout the town, while rumors, fed by Warren, spread throughout The Patch that Jack has started the fire to burn The Patch out. After Bob tells Dion that Daisy caused the fire, they try to warn Jack of the mob that has formed to get him, and although Jack hits Dion upon seeing him, the three brothers are soon united in trying to keep the fire on the South side of the river away from the gas works. As Jack, defying the mob, lights a fuse to dynamite a building in The Patch and make a fire break, Warren's bodyguard shoots him. The subsequent explosion causes the cattle to break out of the stockyards, and as they race through the streets, Warren is trampled. On the South shore of the river, among countless homeless people, Dion finds Belle, who has saved Molly. When Belle turns away from him, Molly berates her until Belle hugs him. As they watch the fire in the distance, Dion and Molly affirm that the dream of Patrick and Jack to see a great city built will be fulfilled.