This is basically the love story of The Doctor and Riversong. A romance running out of sequence. There's even a line taken from doctor who episode when two Henry's are in Claire's bedroom and she asks the older one why he can't remember this happening, and he points to the younger one saying 'I clearly wasn't paying attention.'
Besides this being Moffat's opportunity to play the Doctor and River's on going love story a little longer, I'm enjoying it loads. Great writing, acting, directing and editing.