During season 5 – after “Into the Woods” but before “Crush” – Buffy has a run in with a demon and somehow can’t access her powers. She’s injured and helpless. Spike helps her escape the demons hunting her.

Somehow they end up underground and trapped. Buffy can’t fight back, and Spike can. The pair and the demons stand at a stalemate, separated by a wall of rock Spike built for safety. The demons give Spike one chance: walk away. They know he hates the Slayer and wants her dead.

What makes things worse is that there’s a hole right at the top of their hiding space, meaning that when the sun comes up it will shine through and kill Spike. Buffy’s powers will eventually come back to her, but only after sunrise.

With all of this in mind, Buffy thinks that Spike will abandon her. But he doesn’t. He stays with her the whole time, never even considering the offer, ignoring the taunts from the demons and not explaining why to Buffy.

When the sun comes up and Spike still refuses to leave Buffy’s side, she begins to suspect why he’s staying with her. When her powers come back the two are able to fight their way out.

Things go pretty off-canon after that…