David Merrison was the youngest of four siblings and a sickly child. After his two brothers and sister had been taken by an alien, over three successive Christmases, his father and mother became distant.
When the next Christmas came, David waited for the alien. Before it arrived, the Seventh Doctor and Ace showed up and he told them the story of his siblings' capture. When the alien arrived, it took Ace instead of David, but the Doctor negotiated for the release of Ace and the three children. Once the Merrisons became a family again, David's health began to improve. (PROSE: But Once a Year)