Try to remember every time you have read in a book or manga about a main character regressing back in time wanting to undo some kind of big mistake or horrible future from coming about. Well not this time, this time the regressors won't be able to do anything if the villain/bad guy happens to steal their memories and experiences to change the future for their own personal benefit instead of the regressor.

how do you open the chest also how do you keep the book for yourself instead of giving it to that wizard guy at the tower?