The Castillo family leaves their DNA laboratories for a few days to return home and enjoy a quiet Christmas. Or so they think. But the moment Rosa Ruano knocks on their door with a strange case—the case of the Mischievous Elf—the calm is gone. Borja, now a police officer, needs their help to solve the mystery and earn points within the force.

What begins as an innocent favor quickly turns into a disaster: a compromising video exposes them to the entire country and goes viral in a matter of hours. They soon discover that behind the Elf case lies something far darker: Max, a boy with unsettling powers, is using the doll to manipulate other children and carry out a sinister plan… to destroy Christmas.

With public opinion against them, the police on their heels, and Dante—from ADN—refusing to lend a hand, the Castillos must face not only a supernatural enemy but also their own fears and conflicts. Caught between fame, suspicion, and the most dangerous magic, they embark on one last impossible mission: to reclaim the spirit of Christmas… before it disappears forever.