Now matter how you slice it, David Tennant’s recent engagement to Georgia Moffat is pretty darn strange. It has nothing to do with their age difference or the jealousies of so many single women. Instead it has everything to do with Doctor Who.
In the show, Georgia played the daughter of The Doctor (played by Tennant). In real life, Georgia is the daughter of Peter Davidson, who played the fifth incarnation of the Doctor. That means that either way, The Doctor is marrying his daughter.