Donna Noble had a fairly happy life. She had a mother, a grandfather, and a husband, as well as a two year old daughter. Donna Noble had a family. She was grateful for all she had, and happy with her life, but sometimes she felt as if something was missing from her life. She'd feel an odd sense of déjà vu when she saw a man wearing an unbuttoned trench coat. She'd feel remorse at the sight of a woman sassing their partner in crime. She'd sense a missing piece at Christmas, as though she'd forgotten a family member, even though she couldn't have, her husband's family and hers were all there. But no matter how much her conscious feelings confused her, her dreams were even more perplexing. She'd see a blue light, or hear an old engine, or see a strange creature, and she'd feel a pull towards all of them, but then she'd forget all about it the next day. She knew her family was keeping something from her, they'd exchange looks behind her back, and whisper behind closed doors. But Donna Noble didn't mind. Donna Noble didn't remember.
Wilfred Mott knew what Donna had forgotten. He remembered the man in the unbuttoned trench coat with the messy hair and the crazed expression. He recalled the smiling face hiding a tragic past. He knew the missing piece at Christmas dinner, the spot that should've been left open for the innocent man who'd saved his granddaughter's life innumerable times. He dreamed of the sonic screwdriver unlocking a metal door, the TARDIS whooshing in and out of visibility, the aliens infiltrating Earth. He glanced at Donna's mother with the memory of when Donna had remembered The Doctor, and whispered questions to make sure Donna hadn't remembered anything. Wilfred Mott knew her past. Wilfred Mott would always remember.
The Doctor sometimes remembered the redhead with a fiery temper. He smiled at the times she'd yelled at him, but then they'd disappear from his mind. He was the sad man with a box, and he would mourn the death of a different ginger, as she was the first person this self had seen. Memories of Donna would flash, then be replaced with a new, happier one. He would never forget his companions, they raced into the TARDIS with excited faces, ready to see the universe, and the lucky ones would exit sad, despondent, but alive, while the unlucky ones would never have knowledge it was their final trip, and end up alone, brokenhearted, or dead. The Doctor felt bad. The Doctor remembered.