Our take
The first half of a farewell, and it plays like one: grand, strange and shadowed by an ending everyone knew was coming. The Christmas setting is a backdrop to dread rather than cheer. Bernard Cribbins's Wilf gives it its warmest scenes, John Simm's Master its wildest, and it only really makes sense watched with Part Two, first shown on New Year's Day.
What happens
The Ood warn the Doctor that something terrible is coming, but he fails to answer their summons, and on Christmas Eve his old enemy the Master is reborn. The resurrection, which uses Lucy Saxon, goes wrong: he returns feral, dying and ravenous, able to leap high into the air and fire bolts from his hands. Across the world people are dreaming of his face. Wilfred Mott, Donna's grandfather, is reunited with the Doctor, and his friend Minnie Hooper takes a shine to the Time Lord; in a quiet café scene a tearful Wilf begs him to help Donna, who has no memory of her travels with him. A mysterious woman speaks to Wilf in riddles. The billionaire Joshua Naismith kidnaps the Master, believing he can make the Immortality Gate, salvaged Torchwood technology, give eternal life to his daughter, while two cactus-like Vinvocci work on the Gate in his basement. The Master seizes it and turns every human on Earth into a copy of himself; only Donna resists, and Wilf is shielded inside a radiation-sealed chamber. A narrator in Time Lord robes speaks of the Time Lords' return, as the story heads for its New Year's Day conclusion.
Can the kids watch it?
Best from about ten; it is darker and more frightening than most Christmas specials and ends on an unresolved cliffhanger.