The Doctor attempts to hide from Christmas when he suddenly finds himself dragooned into helping River Song, who doesn't seem to recognize him at all. She is attempting to recover a very valuable diamond from the head of King Hydroflax.
They instead end up stealing the whole head and find themselves being chased by the king's robot body. They eventually find themselves on a space cruise ship full of very bad people, and River attempts to sell the diamond (still in Hydroflax's head). Unfortunately, the buyers happen to be loyal devotees of Hydroflax.
The robot body also catches up with them and quite logically determines that it needs a new head, vaporizing what's left of Hydroflax. It decides that the Doctor's head would be best.
At this point, the Doctor finally manages to convince River of his identity, and they formulate an escape plan. The cruise ship crashes on a nearby planet, Darillium. The Doctor, having recovered the diamond, gives it to a local construction worker and advises him to use the reward money to build a restaurant in sight of the Singing Towers, a pair of natural rock formations.
The Doctor skips ahead in time and reserves a table at the restaurant for him and River, and they share a romantic time. He gives her a sonic screwdriver, knowing that this would be the last time they spend together before she goes to The Library, where she will die.