Lady Alice wanted to control this guy and that is why she chose to use chastity device to do it. She was sure that he would not guess what she wanted to do to him and he did not. The mistress teased him and got him to think that she was interested in having fun with him when in fact all she wanted was for him to wear it and he humiliated. When he wore it, she hid the key and she used the device to control him.