The film begins in New York City, where Benjamin Siegel, known for his brutal ways as an organized crime boss, makes a ruthless statement by killing one of their less significant associates for embezzling from the mob. This act, carried out in front of the man's employees, solidifies Siegel's formidable reputation. Often referred to as "Bugsy," a nickname he detests, he is soon dispatched to Los Angeles to negotiate with fellow gangster Jack Dragna to broaden their criminal enterprises.
Before departing, Siegel bids farewell to his wife Esta and their two daughters. On his journey, he encounters his irresponsible friend Harry Greenberg who, desperate for cash, seeks financial support. Displeased with Greenberg's antics, Siegel hands him a substantial amount and dismisses him to work on his tan under a UV lamp.
Upon arriving in Los Angeles, Siegel reconnects with his old acquaintance, actor George Raft, who takes him to the set of the film Manpower. There, he catches sight of the alluring Virginia Hill, an extra who initially seems unimpressed with him, leaving him feeling slighted. As Raft drives Siegel through Beverly Hills, he points out the homes of various Hollywood elites, and when he mentions that of opera singer Lawrence Tibbett, Ben impulsively decides to secure Tibbett's residence, ultimately persuading him to sell at a price exceeding its market value.
Shortly after, Siegel meets with Dragna to propose ambitious business ventures. Although Dragna initially rebuffs Siegel's suggestions, the two eventually share a laugh, and Dragna reluctantly agrees to collaborate on the deal, wary of the fallout from Siegel's associates — Charlie "Lucky" Luciano and Meyer Lansky.
Not long into his Los Angeles endeavors, Virginia shows up at Siegel's new home, sparking a passionate romance between them. However, this bliss doesn't last long as he receives troubling news from Dragna about a robbery perpetrated by the small-time thug Mickey Cohen. Siegel decides to confront Cohen and, after a heated exchange, hires him for his skills in enforcement and collection.
Tensions rise at home as Siegel's jealousy over Virginia's past and her relationship with Cohen leads to explosive arguments, such as the heated ashtray incident. Siegel brands Virginia as the essence of his grand ambitions by naming his hotel-casino project after her – The Flamingo.
As Siegel immerses himself in his plans for the hotel, he becomes increasingly erratic, his extravagant spending spiraling out of control. Amidst all this chaos, Greenberg reemerges, seeking protectiveness from Siegel, but his return escalates into violence and betrayal, with tragic consequences.
After significant setbacks, including legal troubles and an affair with the casino's finances, Siegel struggles to maintain control over both his business and his tumultuous romance with Virginia. Their relationship comes to a head when jealousy and deception lead to a bitter fallout.
Finally, as the grand opening of The Flamingo arrives, disaster strikes; a storm wreaks havoc on the festivities, leading to its disastrous failure. In the wake of this loss and Virginia's departure from his chaotic world, Siegel faces inevitable betrayal. He is violently taken down in his own living room, a victim of the very world he helped to build. Shortly after, Virginia's fate is sealed in tragedy, as she succumbs to despair amidst her lost fortune.
Although Siegel's life ends in violence, the legacy of his ambition endures. His investment in the hotel lays the groundwork for Las Vegas's transformation into an entertainment capital, generating billions and forever changing the landscape of American nightlife.
