A scientist had once managed to make a time machine that could send someone back in time but not forwards. If they want back in time however they could return easily. They just had to press a return button on the side of the box.
Unfortunately the scientist could never convince anyone that it worked and so ended up giving his machine away.
He wanted to help people by going back in time and changing them then. By making them work out or study. Or he planned on using one of his serums on them when they were just born so that when he went back they had changed. Whether it was becoming sexy or younger or any thing you could imagine. Unfortunately he couldn't convince his friends these worked either. So he gave them away as well, along with the time machine.
But who had he given it to.