Daniel Francis Thompson was the four-year-old son of Michael and Ann Thompson of London. On 24 December 2006, while on an outing with his parents, he slipped away from them and ran into a street to see a house's Christmas lights. He was struck by a car and taken to the A&E department of St Nicholas's Hospital.
As Daniel lay in a coma, he somehow drew the Doctor's TARDIS into his subconscious mind, which took the form of his bedroom. A mental projection of Daniel, which only remembered wanting to see Christmas, entered the TARDIS and journeyed with the Tenth Doctor to several Christmases in history, including a World War I battlefield in Belgium, a party with guests from all over time and space, and even the very first Christmas in Bethlehem. However, none of these satisfied Daniel, and after returning to the TARDIS, he began to fade away. The Doctor realised what was happening, tracked down Daniel's physical body's location, and helped the doctors at the hospital save the boy's life. (PROSE: Deep and Dreamless Sleep)