We join Amy (Karen Gillian) and Rory (Arthur Darvill), the Doctor's companions from the previous season, on their honeymoon. Unfortunately, the passenger ship they are on is about to crash into a planet covered in strange clouds that screws up their controls. Of course, Amy calls the Doctor (Matt Smith) to save them. He can't use the TARDIS to save them so he lands on a building that seems to control the clouds. Sadly, he finds a bitter old man, Kazran Sardick (Michael Gambon), unwilling to help anyone.
This is where the Christmas Carol story begins. The Doctor goes back in time to Sardick's childhood to figure out where the man went wrong in life. He finds a boy who has had his curiosity and love stomped out by a demanding father. Luckily, the Doctor steps in to help foster the boy's curiosity about the fish that swim in the strange clouds. But where there are fish, there are predators too. After dodging a shark attack (and losing half of his sonic screwdriver in the process), young Sardick is crushed to find that the shark is dying. Unless they can get it back to the clouds, it won't survive. The only way to save the shark is to use an "icebox", a life support chamber that Sardick's father uses to control people, to transport it.
Young Sardick chooses the chamber of a young woman, Abigail Pettigrew (Katharine Jenkins), because she loves the fish. Just after releasing her, the shark attacks again. It is Abigail's singing that soothes the shark into submission. Once they save the shark, the boy decides that this has been a grand adventure. As Abigail returns to her chamber, young Sardick promises that the Doctor will return every year to celebrate Christmas Eve with them.
Years pass and the Doctor makes good on the promise. Every Christmas Eve, they release Abigail and travel around time with her. On the seventh year, Abigail confesses to a now teenaged Sardick that she was terminally ill when she was locked in the chamber. She only has one more day to live. Devestated, the lovestruck Sardick locks her back in the chamber without telling the Doctor about her condition. Realizing that life isn't fair, he still continues on the path to becoming a bitter old man.
Amy appears as a hologram to old Sardick in order to show him how to passengers of the doomed ship are coping. Since they know that singing can have an effect on the clouds, all of the passengers attempt to sing carols to stop the ship from crashing. The sound of the singing begins to break through to Sardick, but he just waves it away stubbornly.
As a last resort, the Doctor appears again telling old Sardick that he has shown him the past and the present. Now it is time for the future. He brings the boy to see what a crotchety old man he has become. This has a monumental effect on old Sardick, who vow to use his machine to save the ship. However, the Doctor has changed Sardick so much that the machine no longer recognizes him. Their only option is to use Abigail's voice to control the clouds again.
Knowing that this will be her final day, old Sardick opens her chamber one last time. She sings into the sonic screwdriver and the clouds stabilize, saving the ship and her 4000 passengers. Amy and Rory reunite with the Doctor, while Sardick and Abigail spend their last moments riding around the clouds in a shark-drawn carriage.
I usually like the Christmas specials. The Runaway Bride being the one episode I hated. This time around, I absolutely loved the episode. The interaction between the Doctor and the various incarnations of Sardick were wonderful. My favorite parts were when teenaged Sardick asked the Doctor for love advice. While I enjoyed CChristopher Eccleson and David Tennant as their respective Doctor, I really enjoy Matt Smith's version. He has the ability to be completely fun-loving yet deadly serious at the same time. If you haven't watched Doctor Who recently, this may be the one to latch onto.