At its core, the "Kidnapped By The Mistress" trope subverts traditional power dynamics. In standard abduction tales, the captor is usually a brute or a monster. But the Mistress is different. She is sophisticated, wealthy, and emotionally complex. Her silk gloves hide iron fists.
Readers will reject a purely evil female captor. In Chapter Two, show a flashback of her betrayal. Make us pity her before we fear her. Kidnapped By The Mistress