The King Follett sermon or 'discourse' was a speech made by Joseph Smith shortly before he was killed. The speech was not written but extemporized, and was taken down by various observers - and therefore the primary record is a parallel text by multiple hands:
King Follett is non-canonical for the LDS church - being very obviously a kind of 'thinking aloud', a philosophical speculation on the apparent implications of Mormon theology (in other words, the Prophet was not prophesying at this moment).
I find it a wonderful speech - and it elucidates for me the interplay between what is assumed and fixed in Mormonism, and what are possible consequences of these assumptions, but which are not of the essence.
This is the standard edited text.
which is engagingly dramatized here:
The discourse reveals that the aspects of Mormon theology which seem strangest (and attract most horror and ridicule) are in fact a (speculative) consequence of following up several steps of implications from the primary assumptions of the nature of God and His relationship to Man.
Probably, all metaphysical systems contain infinities - in Classical Christian theology the infinities are given to God - creation from nothing, omnipotence/omniscience, omnipresence and the like. The basic metaphysic is one of statis.
For Mormonism the God of the Bible has none of these attributes; and God is our loving Father primarily and as literally as possible.
The infinities are pushed back and back, until they are out of the realm of our concern altogether - an infinite regress of other Gods in other universes unknown - which are logically implied, but are nothing to do with us in this world, with one God. The monotheism is what matters to us, here now and forever; the polytheism is an answer to a philosophical question.
Another thing to look out for in King Follett is the dynamic nature of Mormonism (in contrast to stasis). The condition for God and for Man is one of eternal progression (another abstract infinite - no bound can be put to progression, exaltation, glory - in a particular sense, not even for God).
But since dynamism is nonsense if everything changes - progression also implies a stasis, against which progress is measured. Thus the necessary eternal existence of matter and laws of the universe 'within which' God works and progresses. Instead of creation from nothing, the Mormon view is that the primary things 'always' existed (from eternity) and always will exist, being re-organizable but indestructible.
In sum, this represents the final stage in a truly amazing theological achievement - one which quite simply, and therefore triumphantly, solves many of the most obvious and troubling - and, I believe, ineradicable - theoretical problems due to the conflict between classical philosophy and Christianity.