Basically the companion character starts off by arriving in London for her sister's wedding. She encounters the Doctor briefly and then is shortly abducted by the bad guys. He catches onto their energy trail or something and manages to rescue her but she heard them talk about getting the other one and they figure out that "the other one" means her sister but they're too late to stop her from being kidnapped (whatever the bad guys are up to they need siblings or relative for it). Then they have to rescue the sister and her finance (who has been replaced by one of the baddies but the actual fiance has been kept alive), and get them to the church on time and save the world etc. Later during the reception the Doctor could come back and spirit her away. Just an idea for a starting point.