Traditional Assumption

According to the traditional story, in September 1823 the angel Moroni directed Joseph Smith to a set of gold plates that he, Moroni, had placed in a stone box in the Hill Cumorah when he was still a mortal around 421 A.D. In September 1827, Joseph was allowed to remove the plates from the box. He took them to Harmony, PA, where he translated most of them (Book of Lehi through Book of Moroni). Then he went to Fayette and finished the translation from 1 Nephi through Words of Mormon. At that point, he returned the plates to the angel, who later showed them to the 3 witnesses. A few days later, Joseph showed the plates to the 8 witnesses near his parents' home outside of Palmyra. Again, Joseph returned the plates to the angel and they haven't been seen since.

The traditional story is fine as an overview, but it leaves some inconsistencies that can be better explained by the new paradigm. 

Here are some examples.

1. The Title Page, which Joseph said was a literal translation from the last leaf of the plates, describes the contents of the plates as two abridgments, plus Moroni's sealing. It says nothing about the original plates of Nephi.

2. Joseph translated the Title Page, which he later said was on the "last leaf" of the set of plates, before he left Harmony. The traditional narrative does not explain why he translated the "last leaf" before he translated the plates of Nephi that were supposedly in the set of plates. 

3. Joseph said the title page was from the "original Book of Mormon."

4. Different witnesses described the plates as 4" thick and 6" thick, 30 pounds, 45 pounds, and 60 pounds.

5. No one can explain where the unabridged plates of Nephi fit into the record if they were included in the plates Moroni gave Joseph.

6. Before leaving Harmony, Joseph gave the plates to a messenger. While traveling to Fayette, Joseph, Oliver and David met a man walking along the road. When David offered him a ride, the man declined, saying "I'm going to Cumorah." David did not know what Cumorah meant. He asked Joseph. Joseph said the man was a divine messenger who had the plates and was one of the Three Nephites.

7. The Lord told Oliver "I would that ye should continue until you have finished this record, which I have entrusted unto him. And then, behold, other records have I, that I will give unto you power that you may assist to translate. (Doctrine and Covenants 9:1–2) In the next section, the Lord told Joseph "you shall translate the engravings which are on the plates of Nephi." (Doctrine and Covenants 10:41)

New Paradigm

After assessing all the evidence, it appears that the set of plates Joseph originally obtained from Moroni's stone box were the abridged plates described in the Title Page and did not include the plates of Nephi. 

That is why the Lord promised Oliver he would "assist to translate" "other records." Then, in the next revelation (D&C 10), the Lord explained that Joseph and Oliver would "translate the engravings which are on the plates of Nephi." These plates were the "other records" that the Lord referred to in Section 9.

Before he left Harmony, Joseph gave the plates to the divine messenger who returned this set of plates to the repository in the Hill Cumorah. From the repository, the messenger picked up the original plates of Nephi and took them to Fayette so Joseph could translate 1 Nephi through Words of Mormon.

Implications

Once we understand that there were two sets of plates, we can reconcile the different accounts and make sense of these details of Church history.

We also understand that the repository Mormon referred to in Mormon 6:6 was actually in the Hill Cumorah in New York, as explained by several of the men who knew Joseph Smith, including Brigham Young, Wilford Woodruff, Heber C. Kimball, and Orson Pratt.

Just remember it this way.

There is one Cumorah and two sets of plates.

Not two Cumorahs and one set of plates.