Moffett, 26, is the daughter of former Fifth Doctor Peter Davison (whom Tennant grew up watching on Doctor Who) and met Tennant while starring in the Doctor Who episode "The Doctor's Daughter," in which she played Jenny, a female offspring cloned from the Doctor's DNA. If the marriage goes through as planned, this means that two former Doctors will officially become related to one another.
And perhaps most disturbingly, it also means the Doctor's daughter will become his wife. You just have to wonder what Professor River Song thinks about that...