The first part of the story is here.

Yama, the god of death, granted Nachiketas three boons, and for his third boon Nachiketas asked to know the mystery of what happens after death. But Yama hesitated. “This is subtle even for the gods to understand. O Nachiketas, do this for me: ask for something else. Ask for wealth and power and long life.”

“These are ephemeral things,” Nachiketas replied. “Wealth and power and long life fade away and only the path to your house remains. I do not wish what is passing, but that which will endure.”

Then Yama agreed, saying that that which is better and that which is more pleasant are not the same, but are two different things. The wise man chooses that which is better, but the stupid man, blinded by ‘getting-and-keeping,’ chooses the more pleasant.

Yama then explained to Nachiketas that the Atman, the soul, smaller than the smallest and larger than the largest, is formless and all-pervading. It is the same as the omnipresent Brahman, and it never dies. The wise strive only to know the Atman. The Atman is the rider, and the senses are the horses; the Atman guides the horses through the maze of desires. No amount of learning or reading of the scriptures will bring the realisation of the Atman; one must learn to discriminate the Atman from the body, which is the seat of desire.

Having heard this instruction from the god of Death, Nachiketas returned to the home of his father a mukta, a fully realised, liberated soul.