JOEL - The only sure thing we know about what happened to the plates is what Joseph Smith himself said as recorded in Joseph Smith History 1:60; "But by the wisdom of God, they(gold plates) remained safe in my hands, until I had accomplished by them what was required at my hand. When, according to arrangements, the messenger(angel Moroni) called for them, I delivered them up to him; and he has them in his charge until this day, being the second day of May, one thousand eight hundred and thirty-eight."
Another report from Brigham Young tells how Joseph Smith and Oliver Cowdery returned the plates to a cave in the hill Cumorah that had a large room containing many plates.
"The first time they went there the sword of Laban hung upon the wall; but when they went again it had been taken down and laid upon the table across the gold plates; it was unsheathed, and on it was written these words: "This sword will never be sheathed again until The Kingdoms of this world become the Kingdom of our God and his Christ"' (JD 19:38). All we know for sure is that Joseph Smith gave the plates back to the Angel Moroni when he was finished with the translation. We don't know what the angel Moroni did with them. We are told that some time in the future we will again receive the plates so that the sealed portion can be translated.