No spoilers here. The action is set on a space cruise ship named Titanic, and the cruise is to Earth in modern day, but the ship is decked out as a replica of the original Titanic, with all the guests and staff in period dress. The set is entirely too small to carry off the illusion, but they make a valiant effort.
David Tennant is The Doctor, he is companionless, but Kylie Minogue plays the part of a cocktail waitress who acts in that capacity for the duration. She's a good actress, I was impressed. The chemistry between her and Tennant is remarkable. Also remarkable is veteran actor Geoffrey Palmer as the captain of the ship. You may not know the name, but you'll recognize his face as soon as you see him.
From the name of the ship, you can guess this will be a disaster story, and it is, but it's not of the innocent "oops, we hit a patch of ice" variety. Written by Russel Davies, it features cardboard characters, a plot which seems to have been written on napkins at several different liquid power lunches, and gratuitous mocking of fat people (a short step away from his usual fart jokes).
Not one of the better episodes, but still worth buying the DVD.