In the tunnels below the ship, Amy questions the Doctor about what will happen to her. The Doctor explains the Nanocloud will erase her memories, "replace her love with hate", and transform her into a Dalek puppet.

Which companion turned into a Dalek?

The Doctor enters Oswin's chamber and discovers to his horror that she is a Dalek. Oswin is revealed to have been captured by Daleks after the Alaska crashed on the Asylum and, to preserve her genius-level intellect for Dalek use, was turned into a Dalek.

Why does Amy Pond not remember the Daleks?

It was the fact that the 11th Doctor's companion — Amy Pond (Karen Gillan) — did not remember the Daleks. This was eventually revealed to be the result of a crack in time which was caused by the TARDIS exploding in the future.