Alma 12

I believe that the 12th chapter of Alma provides one of the most clear explanations of the plan of salvation. It is in this instruction to the people of Ammonihah that Alma teaches -

"Therefore God gave unto them commandments, after having made known unto them the plan..." (Book of Mormon | Alma 12:32)

I intentionally did not complete that verse with "of redemption" because the first plan he gave to Adam and Eve was -

"Be fruitful, and multiply, and replenish the earth, and subdue it..." (Pearl of Great Price | Moses 2:28)

At the same time He -

"...took the man, and put him into the Garden of Eden, to dress it, and to keep it.
16 And I, the Lord God, commanded the man, saying: Of every tree of the garden thou mayest freely eat,
17 But of the tree of the knowledge of good and evil, thou shalt not eat of it, nevertheless, thou mayest choose for thyself, for it is given unto thee; but, remember that I forbid it, for in the day thou eatest thereof thou shalt surely die." (Pearl of Great Price | Moses 3:15 - 17)

In my mind, this was the plan. Adam and Eve had there agency to choose to remain in the garden or be cast out and fulfil the part of the plan in multiplying and replenishing the earth...but in this case they would also have to die. Adam and Eve opted, with influence from Satan (because Satan had his interests at stake), to choose or use their agency to multipy and replenish the earth. And with that - die for -

"...we see that death comes upon mankind, yea, the death which has been spoken of by Amulek, which is the temporal death; nevertheless there was a space granted unto man in which he might repent; therefore this life became a probationary state; a time to prepare to meet God; a time to prepare for that endless state which has been spoken of by us, which is after the resurrection of the dead." (Book of Mormon | Alma 12:24)

Herein lies the second plan, or original plan, or plan of redemption -

"...which was laid from the foundation of the world, there could have been no resurrection of the dead; but there was a plan of redemption laid, which shall bring to pass the resurrection of the dead, of which has been spoken.
26 And now behold, if it were possible that our first parents could have gone forth and partaken of the tree of life they would have been forever miserable, having no preparatory state; and thus the plan of redemption would have been frustrated, and the word of God would have been void, taking none effect." (Book of Mormon | Alma 12:25 - 26)

"Wherefore, he gave commandments unto men, they having first transgressed the first commandments as to things which were temporal, and becoming as Gods, knowing good from evil, placing themselves in a state to act, or being placed in a state to act according to their wills and pleasures, whether to do evil or to do good—" (Book of Mormon | Alma 12:31)

The first set of commandments were part of the first plan which placed man in a state to act in righteousness or wickedness. Then with the plan of redemption, the Lord provides His second commandments -

"...God gave unto them commandments, after having made known unto them the plan of redemption, that they should not do evil, the penalty thereof being a second death, which was an everlasting death as to things pertaining unto righteousness..." (Book of Mormon | Alma 12:32)

And this second set of commandments included -

"...God did call on men, in the name of his Son, (this being the plan of redemption which was laid) saying: If ye will repent and harden not your hearts, then will I have mercy upon you, through mine Only Begotten Son;
34 Therefore, whosoever repenteth, and hardeneth not his heart, he shall have claim on mercy through mine Only Begotten Son, unto a remission of his sins; and these shall enter into my rest.
35 And whosoever will harden his heart and will do iniquity, behold, I swear in my wrath that he shall not enter into my rest." (Book of Mormon | Alma 12:33 - 35)

Thus of this second set of commandments, Alma taught -

"...my brethren, seeing we know these things, and they are true, let us repent, and harden not our hearts, that we provoke not the Lord our God to pull down his wrath upon us in these his second commandments which he has given unto us; but let us enter into the rest of God, which is prepared according to his word." (Book of Mormon | Alma 12:37)