At the culmination of one of his adventures, the Doctor ends up crashing in England just before the Second World War. He’s wearing a spacesuit with the helmet on backwards so that his face is obscured. His kindly rescuer, a woman named Madge, helps him find the TARDIS. In thanks, he tells her to call on him whenever she needs help. All she has to do is wish it. A few years down the road, Madge’s pilot husband ends up dying as his plane goes down. She doesn’t tell their children, fearing it will ruin their Christmas. Madge makes her silent wish and bundles up the kids for a trip to some uncle’s country estate, which is being cared for by the Doctor posing as the caretaker. He’s spiffed up the place with all kinds of wonderful toys and hammocks and gizmos to delight the kiddies. One of the treats is a box that leads to a distant planet and a forest full of natural Christmas trees. Madge’s son Cyril ends up crawling through the box before he’s supposed to and finds himself trapped by some people made of wood.
The Doctor and Cyril’s sister Lily go looking for him, and then Madge goes looking for them. While the Doctor and Lily encounter the tree people, Madge meets some folks from the military industrial complex of that planet and learns that these trees are about to be converted into a fuel source. This involves melting them with acid rain, of which the trees are aware and eager to escape. This is why they’ve lured Cyril to them, but then they realize that Cyril is too “weak” to carry their “life force” off the planet. Much the Doctor’s chagrin, they also determine that he’s too weak. Lily, they decide, is strong, but too young.
Cue Madge’s entrance, as she somehow manages to drive a giant robot and find her offspring. Madge is just the vehicle — or “mother ship” as the Doctor puts it — that the trees need. She carries them off to the time vortex and then manages to get everyone else back to Earth under the Doctor’s guidance. He tells her to wish super-duper hard for home, but she inadvertently brings up an image of her husband about to crash to his fiery death. Her children realize she’s been keeping a secret from them, but lo and behold, she’s also somehow managed to guide her husband’s plane safely home, as well. Everybody lives! Madge realizes the Doctor is the spaceman she helped years earlier. With Madge’s encouragement, the Doctor decides to pop in on Rory and Amy for a Christmas visit. There are hugs and happy tears and a full weecap to come.