You have forgotten everything that happened after your childhood. We met when you were still a child, living in Suwa... after that, you came with me to Shirasagi, staying there to serve me. But some time ago, I sent you to a journey with four others, and on that journey, you lost your arm.
[It is by no means a comprehensive explanation, but it is a start-- besides, she knows he will fully believe it only after he gets back the memories. Because that is how he is.]