Amnesia is a nicely cinematic condition which you can make use of in games, as the result of a blow to the head, magical effects, or insanity induced by beholding things Man Was Not Meant To Know.

Of course, the player* will remember stuff, so it's important to have them play their character with an appropriately frayed memory.

* At least, some players anyway. Character amnesia can actually work better with players who tend to forget their character stories.

Transcript

Han: Where-a am I? How did I getta here?
Jabba: So, Captain Solo, you're not dead after all.
Han: Captain Who?
Leia: He means you.
Jabba: This will make slicing you up so much more entertaining.
Han: Whatta for you slice-a me up? Do I looka like a pizza?
Jabba: Profit. And you soon will.
Boba Fett: You refused to talk. I shall find the whereabouts of Obi-Wan Kenobi from your sliced brain.
Han: Obi-what Ken-who-bi?
{beat - stunned silence as Boba Fett realises the implications of this}
{beat}
Boba Fett: Noooooo!!