Marty Faranan is an aspiring writer hoping to complete his screenplay titled Seven Psychopaths. Struggling with creativity, he finds himself supported by his best friend, Billy Bickle, an actor with a dubious career who resorts to the eccentric and illegal method of dog kidnapping to earn a living, returning the pets for cash rewards. Assisting them is Hans Kieslowski, a devoted man plagued by the illness of his wife, Myra. When Billy suggests incorporating the notorious "Jack of Diamonds" killer into Marty's script, he unwittingly pulls them deeper into chaos.
As events unfold, Marty creates a narrative about another psychopath known as the "Quaker," who obsessively seeks revenge on his daughter's murderer, leading to a journey of madness and despair. The plot thickens when Billy and Hans steal a Shih Tzu named Bonny, not realizing she belongs to the fierce gangster, Charlie Costello. In a reckless stunt, Billy invites "psychopaths" to share their wild tales for their project, drawing the attention of Charlie's violent henchmen, including Paulo, who quickly connect the dots to Hans and the missing pup.
In a tense confrontation at a warehouse, danger looms over Marty and Hans as they cling to their secrets, but fate intervenes with the arrival of the Jack of Diamonds himself, who dispatches Charlie's thugs. As the story spirals, they encounter Zachariah Rigby, another "psychopath" who is eager to contribute his shocking tale, highlighting the increasingly convoluted world they have unwittingly entered. Tragedy strikes as Charlie, relentless in his pursuit, tracks down Myra in the cancer ward and eliminates her when she refuses to divulge anything about Bonny's whereabouts.
Meanwhile, Billy tries to reconnect with his girlfriend Angela, also caught in the crossfire since she is involved with Charlie. The tension rises, culminating in a startling revelation as Billy casually admits to Angela that he is the very Jack of Diamonds they had been writing about, leading to her violent end.
With the shadow of Charlie looming, the trio escapes with Bonny, setting out into the desert. In a revelation, Hans shares his past connection to the "Quaker," which Marty had unknowingly incorporated into his screenplay. As the layers of deception peel back, Billy materializes as a true menace, igniting a series of events that thrust them all into a real-life confrontation with Charlie.
Charlie arrives armed only with a flare gun, escalating the already combustible situation. A chaotic showdown ensues, as Billy struggles with his own choices and their consequences. Amidst the gunfire, Hans embraces a poignant moment before succumbing to injuries, uttering profound words that reaffirm his beliefs. The tension culminates with police ambushing the scene, resulting in Charlie's arrest but not without tragedy as Billy faces his own demise.
With Marty recovering from the encounter and finding solace in adopting Bonny, he bravely returns to his writing, inspired by the tumultuous experiences that shaped his life. The film closes with a thought-provoking twist in a post-credits scene where Zachariah, having just watched Seven Psychopaths, hilariously threatens Marty while reflecting on how much he has grown from this chaotic ordeal. As Marty strolls the street with his completed screenplay, the essence of his journey lingers, marked by the absurdity and darkness of his adventures.
