Does God let anything happen that isn't part of His plan?

I asked my religion teacher this question, and she said yes, but that his overarching plan of good triumphing over evil will always happen, regardless of our sins. I have a hard time accepting this. He has a plan for each of us, but since he is timeless, then wouldn't he know exactly what was going to happen? Original sin was NOT part of his plan, but it happened anyway, and every single human being lives with that original sin. Thus, our personal plans for salvation are based on that original sin, as well as Jesus coming to Earth! How could God's plan for salvation include Jesus coming to save us if original sin wasn't part of his plan, but that's exactly what Jesus came to free us from? Does he keep changing it constantly to fit with the happenings in the world, and if so, then some of his plans must contradict each other. He can't have a plan for no sin and then a plan to save us from sin at the same time, since he is timeless and can see all of existence in one moment. Can someone please clear this up for me?