In 2010, a group of Ethiopian Jewish miners uncovers a rare black opal from the depths of the Welo mine in Africa. By 2012, the story shifts to Howard Ratner, portrayed by Adam Sandler, a gambling addict who owns KMH, a jewelry store nestled in New York City's bustling Diamond District. Howard is ensnared in a web of gambling debts, most notably the $100,000 he owes to Arno, his loan shark brother-in-law. His personal life is torn between his estranged wife Dinah (Idina Menzel), who has agreed to a divorce after Passover, and his girlfriend Julia, an employee at KMH and played by Julia Fox.
The narrative intensifies when Howard's business associate Demany introduces Kevin Garnett, a basketball star, to KMH. During Garnett's visit, the coveted Ethiopian black opal that Howard has been eagerly anticipating arrives. Garnett becomes fixated on the opal, believing it will bring him good fortune in his game that evening, and Howard reluctantly accepts Garnett's 2008 NBA Championship ring as collateral.
However, immediately after Garnett departs, Howard takes a risky turn; he pawns the ring and places a six-way parlay bet, wagering on Garnett's excellent performance during that night’s game. Luck seems to be on Howard's side as he lands a $600,000 win. Yet, the tide turns when he is confronted at his daughter's school play by Arno, accompanied by his men Phil and Nico. Arno discloses that he placed a stop on Howard's bet since it was funded by the money he owes him. In a brutal turn of events, they strip Howard naked and lock him in the trunk of his vehicle, forcing him to call Dinah for assistance.
Desperation drives Howard to a party thrown by The Weeknd, where he tracks down Demany, only to learn that Garnett still has the opal, which inevitably provokes Howard's ire. He then discovers Julia and The Weeknd engaged in drug use, culminating in a confrontation with the singer. Betrayed and furious, Howard orders Julia out of his apartment.
After a tense Passover dinner, where Dinah firmly rejects Howard's attempts to salvage their marriage, Garnett returns the opal and offers to buy it for $175,000. Yet, Howard, banking on a much larger appraisal, turns down the offer. As the auction nears, Howard learns with dismay that the opal's appraisal falls vastly short of his initial expectation of $1 million. In a desperate move, he enlists his father-in-law Gooey to bid on the opal to inflate the price. Unfortunately, their plan backfires when Garnett’s final bid fails to surpass Gooey’s, resulting in Howard being assaulted outside the auction house by Arno, Phil, and Nico.
Bloody and devastated, Howard returns to KMH, where Julia comforts him, leading to a moment of reconciliation. Howard later finds out Garnett remains interested in purchasing the opal. Upon the sale's completion, he stakes his winnings on another parlay, betting on Garnett's stellar performance. In a tense scenario, he instructs Julia to fly via helicopter to Mohegan Sun to place the bet on his behalf. As Arno, Phil, and Nico close in on KMH, Julia narrowly escapes.
In a dramatic twist, they confront Howard, but he defiantly refuses to contact Julia to cancel her bet. In a fit of rage, they attempt to follow her, but Howard cleverly traps them between the store's security doors. Watching the game unfold on television, Howard experiences elation when Garnett's Boston Celtics win the game, earning him $1.2 million. However, the high of victory is short-lived; as he releases Arno, Phil, and Nico, Phil, consumed by rage, shoots Howard dead. Arno's protests go unheard as Phil subsequently kills him too. Meanwhile, Julia exits the casino with Howard's winnings, and as chaos ensues, Phil and Nico plunder the store, leading to a chilling conclusion.
