The Katha Upanishad opens with young Nachiketa watching his father Vajashravasa perform a sacrifice in which he gives away old, barren cows to gain merit — a hollow gesture Nachiketa finds troubling. Pressing his father repeatedly, “to whom will you give me?”, he finally provokes an angry reply: “I give you to Death.” Taking the words literally, Nachiketa travels to the abode of Yama, the god of death, and waits three days at his door while Yama is away. When Yama returns to find an uninvited guest left unattended, a serious breach of hospitality, he offers Nachiketa three boons in recompense.
Nachiketa spends the first two boons modestly: a peaceful return to his father, and knowledge of a fire ritual that leads to heaven. For the third, he asks the question Yama would rather not answer: when a person dies, does something of them continue, or not? Yama tries everything to dissuade him, offering wealth, kingdoms, long life, and celestial pleasures instead, calling the question too subtle even for the gods to have settled. Nachiketa refuses every substitute, drawing the distinction the Upanishad becomes famous for, between shreyas (the good, what ultimately benefits) and preyas (the pleasant, what merely gratifies), and insisting that only shreyas will do.
Satisfied that Nachiketa is a fit student, Yama finally teaches: the Atman is never born and never dies, subtler than the subtlest and seated in the heart of every being, and it is known through the syllable Om, the sound the Upanishad identifies with Brahman itself. Yama then offers the text’s most enduring image, the parable of the chariot: the body is the chariot, the senses are the horses, the mind is the reins, the intellect is the charioteer, and the Self is the passenger being carried — reachable only by a charioteer, an intellect, disciplined enough to hold the horses of the senses in check.