At (someone's) mercy phrase
Under the control of or controlled by the acts of another, without the power to protect or liberate oneself.
He pleaded guilty and he was at the court's mercy.
Peter was kidnapped and briefly detained by a terrorist group. He was entirely at their mercy.
A little girl was at a mad man's mercy. We have to hurry up.
To have someone in a difficult situation in which you have complete power over them.
To tell someone the truth about something that they had not understood or received false information
I have to set my wife straight about what happened yesterday.