This story is built on a hugely fun idea - stick the 3rd Doctor, Jo and Mike Yates into the events of The Christmas Invasion. Taking the idea of the Pilot Fish and the murderous Christmas tree from the early parts of the story and developing them into more of a threat (with the original story admittedly dispensing with them fairly early on to focus on the Sycorax) forms the core of this story with some added Jackie Tyler for good effect. It's huge fun, particularly at the beginning when Jackie thinks the Doctor has regenerated again, and this time it's happened to Rose (who she thinks Jo is) and Mickey (Mike). There's also fun to be had with Jo and Mike trying to understand 21st century technology.