They go down to the sitting room where the children have found the Doctor-enhanced Christmas tree along with a large, TARDIS-blue gift box. The Doctor enjoys the sight of the children oohing over his handiwork, then disappears to do some unknown Doctor-y things. Madge: "That man is quite ridiculous! You must stay away from him." Lily and Cyril like him, though, and implore their mother to say what she always says. She indulges them: "This Christmas is going to be the best Christmas ever!" The blue box whispers and glows, but only Cyril notices.
Later, when everyone is tucked into their assorted sleeping surfaces, Lily sneaks away to have a peek at the blue gift. Before she gets halfway down the stairs, she hears an odd sizzling sound coming from the caretaker's room and goes back upstairs for a look. Meanwhile, Cyril sneaks off to investigate the gift. What sneaky children they are. Lily walks in without knocking first. Luckily, the Doctor is just doing mechanical things with the sonic screwdriver and not, say, dancing around naked or doing other grownup things. She sees the TARDIS. "Why have you got a phone box in your room?" she asks. It's where he stuffs the carcasses of nosy children, missy. "It's not a phone box," he says, "it's my... wardrobe." Downstairs, Cyril opens the box and finds it full of light and snow. The other end of the box opens into a snowy forest. Cyril gapes and backs out of the box. Suddenly, some mechanism beeps and the Doctor wonders where Cyril is. Lily goes to check the bedroom, finds a lump in Cyril's hammock and assumes it's her brother. She reports back to the Doctor that Cyril is fast asleep. The Doctor assumes his doodad must be faulty.
In actuality, Cyril is drawn by the whispers coming from the forest. He crawls back through the box and into Narnia. A pine tree oozes some snot from one of its branches, which hardens into something that looks like a silver Christmas ornament. Cyril plucks the ornament from its snot casing. It expands in his hands. He drops it and it grows some more, then splits open. Cyril hurries back through the box. The Doctor's doodad beeps a warning once more. Cyril, perhaps putting in an early bid for the Darwin Awards, goes back into Narnia and finds tiny footprints leading away from the now-shattered ornament.
The Doctor and Lily find that Cyril isn't in his hammock at all and rush downstairs and into the box after him. Lily gawks at Narnia. "Where are we?" she asks. "In a forest, in a box, in the sitting room," the Doctor says. He checks the dual footprints and estimates that Cyril is twenty minutes ahead; time moves differently here, he explains. He's a bit annoyed that this dimensional portal -- his gift to the children -- was opened a day early. I suggest laying out bear traps next time. Those things really keep the kids away.