Also known as roughly the greatest Star Trek TNG episode ever. Everyone's memory is erased, so they don't know who they are or their relationship to each other. But they still have their basic character, and you can see how this plays out without the limitations imposed by their preexisting relationships with each other. It's very well done---I didn't notice any times when people acted as though they had a memory.
All the elements of a great Star Trek episode are present: Data being super analytical about his situation, analyzing all possible origins of himself completely objectively; Worf being a super badass battle-focused Klingon; and Riker getting it on with two women only to be shot down by both once their memories return.
It really sucked me in and got me thinking like I would if my memory wasn't there. Feels kinda like it would be liberating, or something. I'm too sleepy to think hard about why I found this profound, so you'll just have to trust me, it was :P.