The Doctor has the uncanny ability to flummox everyone he meets. All he needs to do is open his mouth and start talking, rambling away in his usual incomprehensible technobabble. Sometimes, Donna is sure he does it on purpose, just to see the baffled expressions of confusion on everyone’s faces.
This time though, it seems the Doctor has met his match; not only has he completely failed to confuse Professor Song, but he doesn’t quite seem to know what he should do about it, so he just gapes at her, utterly bewildered.
For her part, River simply smiles. “Spoilers, Sweetie.”