It is not every day that your dead son turns up on the doorstep, clutching the dead body of the human child you took in after they freed everyone from the Underground.
Sometimes, in the dead of night, there are dreams.
Except, Undyne's pretty sure they aren't dreams--they're a lot more tangible. They're clear and they make sense, and Undyne isn't quite sure what to make of it, because, the thing is, sometimes they're scary. (Not that she'd be scared of anything, a nightmare or otherwise, thank you very much.) And sometimes they're not. They don't always fit together, so they can't be memories, and they aren't real, so that means they must be dreams.
But they're not.
In which Undyne remembers.