Doctor Who Season 4 Episode 13

The Doctor: She took my mind into her own head, but that's a Time Lord consciousness. All that knowledge, it was killing her.
Wilfred: But she'll get better now?
The Doctor: I had to wipe her mind completely. Every trace of me or the TARDIS, anything we did together, anywhere we went had to go.
Wilfred: All those wonderful things she did.
The Doctor: I know. But that version of Donna is dead, 'cause if she remembers- even for a second- she'll burn up. You can never tell her. You can't mention me or any of it for the rest of her life.
Sylvia: But the whole world's talking about it! We traveled across space.
The Doctor: It'll just be a story. One of those Donna Noble stories where she misses it all again.
Wilfred: But she was better with you.
Sylvia: Don't say that!
Wilfred: No, she was!