Once you get past the basics I'm not sure anyone understands the rest of it, so time travel's a reasonable summing up.

[ no no please no hackles, she shakes her head and smiles, a little hopefully and a little bit in attempted reassurance. ]

I know it's a strange concept, but it happens occasionally. Someone goes home and returns without remembering their past stay. It's something to do with the machinery.

[ A pause, because she's not entirely sure how to reassure him that it's all right. ]

I thought you should be prepared, in case others recognized you. And it is - good to see you again.