Welcome to Plan A. Let me introduce you to the outset of this plan -- Adam. On the sixth day, God made Adam. Adam was without sin. He was made in perfect harmony with God. He had a "walk in the garden" relationship with God. Soon thereafter was the second part of Plan A, Eve. Both made in the image of God, both without sin, both without even a sin nature, they constituted Plan A -- a world predicated on beings made in the image of God reflecting the nature of God to all creation. And it was very good.
Enter the serpent.
"More crafty than any other beast" (Gen 3:1), we get Satan (Rev 20:2) tempting Eve with "Did God say ...?" And Eve, seeing that "the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate." (Gen 3:6) Say goodbye to Plan A.
Most people believe that what follows (effectively all of human history) is God's Plan B. His Plan A was an eternal harmonious relationship between Man and God, but we messed that up, so He had to come up with a Plan B which included the dreadful death of His Son, His resurrection, and the ultimate defeat of Satan. Good plan, as far as alternate plans go.
There's a problem with this line of thinking. The Bible disagrees. In Titus, for instance, we read of the "hope of eternal life, which God, who never lies, promised before the ages began." (Titus 1:1-3) Wait, promised before the ages began? So that was the plan from the beginning. In Revelation we read of "everyone whose name has not been written before the foundation of the world in the book of life of the Lamb who was slain." (Rev 13:8) Now, wait a minute. This says that those who are saved had their name written in the book of life before the foundation of the world. That means that 1) all who would be saved were already documented, so to speak, and 2) those who would not be saved were not.
This paints a different picture, then. This argues that the idyllic garden existence was the beginning, to be sure, but was not a Plan A to a Plan B. This argues that it was simply the beginning to Plan A which, in God's good time and for God's good purposes would then take a turn into sin according to His own prior knowledge, and that this was Plan A from the beginning -- our sin, the need for a Savior, the death and resurrection of Christ, perfect redemption.
I know. There are some who will balk at this. It seems to suggest that God knew in advance that Adam would sin and bring sin into the world and that this was His plan. If that's what you think, then I'll go along with you there. If God is Omniscient, it is the only thing we could conclude. He knew it would happen, could have prevented it, and didn't. Instead, He intended to show His power and wrath and grace and mercy (Rom 9:22-23) for His glory. Without authoring -- causing or putting into place -- sin, He allowed it into His Plan A along with the remedy, "promised before the ages began." The alternative to this is that God is not Omniscient -- didn't know it was coming -- or not Omnipotent -- knew it but was incapable of stopping it.
As for me, I take a great deal of comfort knowing that we are not on Plan B, God's back up plan for when Plan A failed. Instead, He continues to cause all things to work together for good. It would be comforting to know that He could remedy any misstep, I suppose, but knowing that it hasn't been necessary gives me greater confidence in His Omniscience, His Omnipotence, and His Sovereignty. I don't have a "Plan B" God. I trust a God who always had it under control and will continue to have it under control. It works for me. You can decide for yourself.