The premise is this:
JS and his friend Oliver Cowdrey were translating the BOM. At one point in the translation, JS translated an event in which men were baptizing people into the church of God. He was suddenly caught up with the thought that he will be baptizing people into the church of God, but he had never been given the right or authority to baptize.
So with that thought in their minds they went into a wooded area and knelt down to pray about what authority was needed to baptize. At that time an angel came to them and he said that his name was John the Baptist, the same John that ministered and baptized the Savior. He was sent from the presence of God to Joseph and Oliver to restore the Aaronic priesthood again to the earth. So he laid his hands on their heads and bestowed the Aaronic priesthood to them and with that authority, they were now authorized by the Lord to baptize people into the church of God.
Some months later, JS and Oliver were again praying to the Lord, and He sent Peter, James, and John to them, who were apostles of Jesus Christ. They too, laid their hands on their heads and bestowed the Melchisedec priesthood to them, which gave them the power and authority of the Melchisedec priesthood, which is the power and authority to preside over all the affairs of the church.
So the premise is that priesthood power was given to JS by heavenly messangers sent from God and JS now had the power and authority to do what was necessary to operate the church according to the will of God.
If JS does not recieve priesthood authority it is all over.