Fancy a trip in the TARDIS? The Doctor has dropped off Rose at an ABBA concert at Wembley in 1979, so he’s going to need your help. First, he’ll connect your digital remote control to his sonic screwdriver, and then you can help him find out what’s gone awry with an ordinary family Christmas. Mum and Dad, gran and granddad, and a little boy and girl are milling about the living and dining room, going through their presents and preparing to eat while the girl tests out her new camcorder and tries to get all the family in one place for her Christmas video. If you watch the scene carefully, you’ll notice that the mother’s eyes briefly glow yellow -- but why? Moments later, the mother calls the father into the dining room -- and an alien materialises on the dining-room table and zaps the dad with a hand-held control, replacing him with an exact duplicate whose eyes briefly glow yellow.

The Doctor identifies the alien as a Graske, one of a species that takes over planets by replacing their populations with duplicates. While the Doctor tries to follow the Graske’s trail, you must pilot the TARDIS for him, using your remote control to operate the dimensional stabiliser, the vector tracker, and the vortex loop when the Doctor tells you to. The trail leads back in time; you must keep an eye on England from orbit, and select the section of the grid where the Graske’s DNA trail appears. The trail leads to a street in London on Christmas, 1883; if you look closely, you’ll see exactly where the Graske is hiding. The Graske then leaps out of hiding, and in the ensuing confusion and panic, he zaps a young street urchin and replaces him with a duplicate.

The TARDIS pursues the Graske to Gryffoth, its home planet. The entrance to the Graske’s base is shielded, and in order to get inside, you’ll have to solve a series of three logic codes; two are shape-recognition puzzles, and one is numerical. Once through, you find yourself in the chamber where the Graske store their victims’ bodies; they need to keep the originals alive in order to sustain the copies. As the Graske transfers his two latest victims into the cubicles, he spots you and opens fire -- and when you duck out of the way, the ricochet blasts open another cubicle and releases an angry Slitheen. The Slitheen pursues the Graske through the chamber, giving you a chance to reach the controls, but you only have time to do one of two things: reverse the settings, which will destroy the duplicates and send the originals back home; or use the stasis controls to freeze everything except you in place.

This is the tricky bit. If you choose to operate the teleport, then well done -- all of the Graske’s victims, including the Slitheen, are transported back home. Back in the family house, the proper dad and mum snap back to normal and start having fun with their family, and as the boy plugs in his new video game, the girl prepares to make her Christmas video. Well done; there’ll always be another chance to deal with the Graske, and in the meantime, you’ve saved Christmas -- and proven yourself a worthy companion of the Doctor. Maybe if he ever needs a new companion, he’ll be by to pick you up one day!

If, on the other hand, you choose to operate the stasis field... that’s not so well done. You’ve frozen Time, so everyone’s staying right where they are... including all of the Graske’s victims. Back on Earth, the duplicated mum and dad sit blandly on the sofa, suggesting that the family go for a brisk jog in the cold or cut their toenails, and they simply stare blankly at the girl when she begs them to do the Christmas video with her. Nice going; you just ruined Christmas and failed to prove yourself a worthy companion of the Doctor. Disappointed, he drops you off back home, but admits that you did okay for a first run and that maybe you’ll improve one day.

But you can always try again...