The Haldenmor Fugue (short story)

Summary
The Doctor lands in a major city in Eastern England in the year 2180. Strangely, the Doctor does not recognise the city. Soon, he finds himself partnered with a strong-willed security guard named Carla, investigating odd happenings in a building where "ghosts" are reported attacking customers.

The Doctor soon realises these "ghosts" are Vikings who settled in the area back in the 10th century, but who are interacting with the 22nd century via a rift in time created by a time travel experiment gone haywire. To put things right, the Doctor must close the rift, but doing so will make it so the city of Haldenmor and possibly his new friend, Carla, never existed.

Characters

 * Tenth Doctor
 * Has taken to talking to himself because of his lack of companionship.
 * Regrets that he is unable to take Carla on as a companion.
 * Carla
 * In the Haldenmor timeline, is a security guard for a major building.
 * In the proper timeline, is a jet pilot named Yarah.

Continuity

 * The Doctor spends years tracking down Carla's whereabouts after having repaired the timeline. Exactly how many years is unrevealed, nor is it indicated whether the Doctor dedicated himself exclusively to the task during this time. The Doctor is still in his tenth incarnation when he finally locates Carla. In retrospect, it is possible to suggest that the Doctor's eventual location of Carla might even take place during his final "rewards" travels before his regeneration (TV: The End of Time, Part Two). Indeed, the story presents his observation of her in a fashion similar to how he observed his other companions.
 * The Doctor discusses his decision not to have companions any more, placing this in the post-Journey's End era.