The Castillo family leave the ADN laboratories for a few days to spend a quiet Christmas together, but everything changes when Rosa Ruano arrives with a new case: the Mischievous Elf. Borja, now a police officer, asks for their help to solve it and gain recognition. What starts as a simple favour turns into a disaster when a compromising video goes viral and exposes them to the entire country.
They soon discover that the case hides something much darker: Max, a boy with disturbing powers, is using the elf doll to manipulate other children and carry out a plan to destroy Christmas. With public opinion against them, the police chasing them, and Dante refusing to help from ADN, the Castillo family must face both a supernatural enemy and their own internal conflicts.
Trapped between scandal, suspicion, and dangerous magic, they embark on one final mission to save the spirit of Christmas before it disappears forever.