They learn that River Song was also invited to this place.

The Doctor returns from another night out with River Song.

And in the words of River Song, you'll see me again quite soon.

And more River Song as well i would hope.

"River Song didn't get it all from you, sweetie."

And, although they do not appear in the episode, River Song mentions them.

River Song was first seen in the series as a future companion of the Doctor.

It was later shot in the eye by River Song.

This sound was best presented on their breakthrough album River Songs, released in 1995.

The character of River Song was originally created for the plot to make more sense.