The central mystery of "The Big Lebowski" is a classic MacGuffin—a plot device that motivates the characters but is ultimately irrelevant. The entire kidnapping plot is a series of deceptions. Bunny Lebowski was never actually kidnapped; she simply took an unannounced trip out of town. The German nihilists, who are friends of Bunny, seize the opportunity of her absence to try and extort money from her wealthy husband.
The biggest reveal is that the "Big" Jeffrey Lebowski is not a victim but a perpetrator. He is not a wealthy man of achievement but is broke and living off an allowance from the trust of his late wife, which is controlled by his daughter, Maude. Knowing Bunny had left, he fabricated the kidnapping narrative, withdrew a million dollars from the family's charitable foundation, and gave The Dude an empty briefcase to deliver as ransom. His plan was to embezzle the money for himself, blaming its disappearance on the non-existent kidnappers.
In the end, literally nothing is resolved in a conventional sense. The money was never in the briefcase and is never recovered. The Dude solves the "case" only to realize there was no case to begin with. The film's climax is not a dramatic shootout but a confrontation where the truth is revealed, followed by an anticlimactic altercation with the nihilists in the bowling alley parking lot. This fight leads to the film's only real tragedy: the death of Donny from a heart attack, an event completely random and disconnected from the main plot. The Dude returns to his life exactly as it was, with no reward and having lost a friend, reinforcing the film's absurdist and existential themes that life is chaotic and doesn't adhere to neat narrative structures.