From the moment he saw her, she knew she was going to die. Silence in the Library is one of the most terrifying episodes of Doctor Who, not only due to the impending threat of the Vashta Nerada (shadow piranhas), but also because The Doctor encounters a woman who knows him perfectly, although he has never met her. River Song’s introduction is a key moment. When Professor Song asks about the crash at the Byzantium, we can see how time travel is
no longer a story-telling device, but a character designed to place these friends in turmoil, especially when River pleads “Doctor. Please tell me you know who I am.” and all he can say is “Who are you?” Her sadness at his not recognizing her is heartbreaking. She clearly loves The Doctor deeply. She realizes she is going to die soon and he will know how for the length of their relationship. This cruel twist of fate shows how difficult life can be for the Doctor’s companions and is brilliantly shown in just a few lines. For the rest of her story arc (which lasts seasons), we watch as she forgets him, just as he gets to know her.