Liam Turner, a polite yet timid sixth-grader grappling with dyslexia, is navigating the challenges of moving to a new town while dealing with a fractured family life. His parents, Bill Turner and Molly, are struggling with their own marital issues after the recent passing of Liam's older brother, Spencer. Amidst this turmoil, Liam battles his own insecurity, particularly when it comes to introducing his new best friend Gibby to his parents, and he often finds himself feeling embarrassed around Emma, the girl in his class who he has a crush on. Notably, Emma is the only one who stands up for him, recognizing his dyslexia and offering him much-needed support.
In a moment of hopeful inspiration, Liam decides to pen a Christmas wish list and sends it off, but due to a comical spelling blunder, he mistakenly addresses it to "Satan" instead of "Santa." To his sheer astonishment, Satan, portrayed by Jack Black, appears in his bedroom that very night. Initially mistaking him for Santa because of his eccentric looks and demeanor, Liam plays along as Satan offers him three wishes. In a moment of teenage impulsiveness, Liam wishes to have a chance with Emma, leading to a whirlwind of events. The next day, Satan appears at Liam's school and encourages him to approach Emma, resulting in a date at a Post Malone concert, complete with VIP tickets courtesy of the devil himself.
Thrilled by his good fortune, Liam confides in his mother and Gibby about his encounter with "Santa." However, Gibby soon connects the dots, realizing the mistake about who Liam really interacted with. When confronted by Gibby, Liam attempts to cancel his wish, but Satan persists, luring him with the enticing prospect of making the most of his remaining wishes, all while his soul hangs in the balance. Concerned about their son's increasingly fantastical claims, Liam's parents summon Dr. Finkleman, a child psychologist, for an intervention. Unfortunately, the session is disrupted when Satan and Gibby sweep Liam away to the concert, where he finds himself on stage dancing with Post Malone, even capturing a kiss from Emma.
While Liam enjoys newfound popularity, his friend Gibby feels overshadowed and leaves early, troubled by his own insecurities. After noticing Gibby's sadness, Liam selflessly uses his second wish to help fix Gibby's overbite, inviting him on an exciting double date with Emma at the local Christmas carnival. Unbeknownst to them, Liam's parents overhear the conversation and, fearing for his mental state, pull him back to Dr. Finkleman. Their worries only amplify when a nurse mentions seeing Liam at the concert, which plays havoc with their beliefs about his so-called delusions.
In the meantime, Satan goads Liam to utilize his final wish by showing him false images of Emma and Gibby enjoying the carnival without him, prompting Liam to lock himself in his room. During this time, his parents, finally confronting their grief, begin to patch things up. Misinterpreting their heartfelt conversation as a sign of impending separation, Liam becomes desperate and visits Satan at his motel, making his third wish to ensure that his parents remain together and in love eternally. Satan agrees, but makes it clear that he won't claim Liam's soul until he dies a natural death, urging him to make the most of his life in the meantime.
With the weight of his perceived doom weighing heavily on him, Liam finds the courage to stand up to his classmates who torment him, albeit at the cost of alienating Emma. Meanwhile, in Hell, Satan is revealed to be a half-demon on trial, and by accommodating two of Liam's unselfish wishes, he has actually invalidated the contract for Liam's soul. This angers Lucifer, who immediately voids the deal and banishes Satan. Granted an hour to bid farewell, Satan visits Liam once more, expressing that he genuinely likes him and revealing that Liam's final wish was unnecessary because his parents were already reconciling on their own. Instead, Satan made a wish from Liam's original letter come true.
On Christmas morning, Liam is left astounded to discover that his brother Spencer has returned, miraculously resurrected, bringing their family back together for the holiday. Just when Liam begins to bask in the joy of this reunion, he receives a call from Gibby, who informs him that he managed to win Emma back by sharing the story of Spencer's death. Caught off guard by the news, Liam experiences a rush of panic but resolves to make amends, setting in motion an emotional journey to reclaim the connections that truly matter.
