Board Thread:The Reference Desk/@comment-5545417-20131116122722/@comment-43908-20131124181537

"The Four Doctors" ends with the earlier Doctors in Eight's TARDIS. They mention the fact that after they part ways and return to their own places in their timeline, only Eight would remember the encounter.

"The Empire of Glass" begins with the Doctor reappearing in his TARDIS after assisting his future selves in "The Three Doctors;" he doesn't remember what happened beyond fading memories of "a dandy and a clown."