The first time he meets her, she dies.
That’s how Doctor Who introduced River Song, in 2008. The Doctor meets a stranger who knows him intimately, who even knows his real name, which he has never told anyone. She dies there, saving him. For him it’s the beginning; for her, the end.
They’re both time travellers, so they meet out of order: roughly, his first time is her last, and her first is close to his last. She carries a diary to work out which version of him she’s talking to, and when he asks about things that haven’t happened to him yet, she answers “Spoilers”.
On screen, in broadcast order:
- 2008: she dies, and he saves her mind in a computer, so she doesn’t vanish completely.
- 2010-2011: we watch them fall in love in the middle of the story. We find out who she is (the daughter of his companions Amy and Rory, raised to kill him), and they marry.
- 2015: one of the first chapters for her, the last one for us. He finally takes her to the Singing Towers of Darillium, the night she’d mentioned the day she died. That night lasts 24 years, and we know what comes in the morning.
That’s why it hurts: the ending is in the first episode. Everything we see afterwards is the beginning of a story we already watched end. He falls for someone he knows he’ll lose, and she spends the relationship watching him know her a little less each time.
“Spoilers” was a joke. It was also the only way she had to protect him.