Alma 42:11 - 42:15
Alma 42: 11 And now remember, my son, if it were not for the plan of redemption, (laying it aside) as soon as they were dead their souls were miserable, being cut off from the presence of the Lord.
If it had not been for the plan of redemption, when we die we would be cut forever from God and as a result would be very miserable because of it.
There is no happiness outside of God presence where in His actual presence or under the influence of His spirit.
Alma 42: 12 And now, there was no means to reclaim men from this fallen state, which man had brought upon himself because of his own disobedience;
And unless something is done, this state of being cut off would last forever because of the actions of Adam and Eve.
Alma 42: 13 Therefore, according to justice, the plan of redemption could not be brought about, only on conditions of repentance of men in this probationary state, yea, this preparatory state; for except it were for these conditions, mercy could not take effect except it should destroy the work of justice. Now the work of justice could not be destroyed; if so, God would cease to be God.
So what is required is that man repents of his sins. It is then that the plan of redemption can have an effect on him. Justice cannot be robbed, so if a man repents then Christ takes the punishment that justice requires. Otherwise the man pays for his own sins himself. And if justice were set aside because we might think it is only fair then God quits being God having lied when He said we had to repent or pay of our own sins.
Alma 42: 14 And thus we see that all mankind were fallen, and they were in the grasp of justice; yea, the justice of God, which consigned them forever to be cut off from his presence.
So the bottom line is man commits sins and as a result is cut off from God. If that state remained, it would last forever.
Alma 42: 15 And now, the plan of mercy could not be brought about except an atonement should be made; therefore God himself atoneth for the sins of the world, to bring about the plan of mercy, to appease the demands of justice, that God might be a perfect, just God, and a merciful God also.
To overcome this problem, the plan of redemption was devised so the God himself pays for the sins rather than the sinner if the sinner repents. In that way justice is satisfied, mercy has been meted out and God has kept His word saying that He will provide a way back to His presence.