Ethan tried to analyze the motive of the person with Sadie. If they wanted to kidnap Saide, they would've taken her away as soon as they got her. They wouldn't bring her here to deliver the baby.

It was clear that the person was trying to help Sadie, which meant they were on the same side.

So, Ethan announced himself and called Sadie by her alias. "Brie, are you in there? Michelle is coming over. I'm here to save you."

Sadie was very excited, but Olivia was even more so. She didn't expect t