In Katha Upanishad the teacher is Yama -the death himself and the student is a young Nachiketa in his teens. This Upanishad, which consists of 120 verses, is one of the most popular Upanishads known for its simplicity and clarity in making the subject matter regarding the Highest Truth easily understandable.
The story goes this way that Vajashrava ,Nachiketa's father decided to perform the sacrificial ritual Viswajit Yajna in which the performer had to give away all his precious possessions .Since cows were held to be valuable and special possessions in those days Vajashrava decided to donate all his cows to the Brahmins. Nachiketa who at that time was in his teens was observing the sacrificial ritual with innocent interest. However he was surprised to notice that his father was giving away only old and disabled cows that were not capable of bearing calves and were of no use. This 'worldly cleverness' of his father produced enormous change in the heart of this young boy in whose heart was now immersed in Shraddha ( Faith). Nachiketa, in order to dissuade his father from engaging in further mean acts, asked, "O father, to whom have you decided to give me away?" (The purport was to bring to the notice of his father the fact that he has to give his everything and not just old cows.)
Initially, the father did not take any notice of this 'childish' question, but Nachiketa was stubborn and he repeated the question thrice when his irritated father said that he gave up young Nachiketa to death. Young Nachiketa went to the kingdom of Yama - The Death - where he waited for the return of Death from his duties. On his return, Yama was told about the 'Brahmin boy waiting for him for there days without food or water'.
Yama was impressed with the sincerity of Nachiketa, but also felt grieved that he was responsible for keeping a Brahmin boy waiting for him. Therefore, he granted three boons to Nachiketa one each for a day of waiting.
For the first boon Nachiketa asked that his father may become freed from anxiety about him and he may recognize him and talk to him when freed .The boon was granted. As the next boon Nachiketa asked for granting him the knowledge of the means to attain higher life in the heaven and immortality. In the Katha Upanishad there is a dialogue between Yama and the boy about the pure Fire and sacrificial rituals to attain heavenly life. Yama tells him about the methods and ways of performing these Yajnas etc. Death tells him that it is the sharpened intellect of the enlightened one, which qualifies that sadhaka to become fit to go to heaven. In heaven there is no fear, fear of old age, etc. Having overcome both hunger and thirst, and crossed over sorrow, one rejoices in the heavenly world.
Then comes the main subject matter of this Upanishad. Regarding the third boon, Nachiketa wanted to know about Truth itself .He asked for the ultimate knowledge. What is death, what is after death! What is Reality and what is Truth? Yama tries to dissuade the young boy from going into these complicated questions which have various shades for Death was not sure whether Nachiketa was qualified to receive this knowledge for which only an occasional and rare aspires.
Yama persuades Nachiketa to ask for health, life, riches, jewels, and enjoyment,lasting kingdom and armies and promises to grant everything as his third boon but requests Nachiketa not to force him to go into the secrets of life and death-the ultimate knowledge. He also promised to grant him sons and grandsons, elephants horses and vast expanses of the earth as well as live for as many years as he wants.He insisted on asking for any delectable things that fits his enjoyment .
He requested him not to inquire about death but to ask for anything even if it is difficult to get .He asked him to pray for all those cherished things according to his choice.
But Nachiketa argues that all worldly treasures and heavenly pleasures come to an end sooner or later and these are not permanent means of enjoyment. Nachiketa insists on knowing the ultimate knowledge of the self.
Seeing the determination, faith, sincerity, and perseverance of Nachiketa, seeing him to be the perfect disciple, Death agrees to tell him about the Ultimate Reality: Brahman or Atman.
Death says that the Atman and the Brahman serves divergent purposes and bind men. "Good befalls him who accepts the preferable among these two". One who selects pleasurable over preferable falls from the true end.
"The preferable and pleasurable approach man. The man of intelligence, having considered them, separates the two. The intelligent one selects the electable in preference to the delectable; the non-intelligent one selects the delectable for the sake of growth and protection of the body, etc." I. ii. 2.
"Living in the midst of ignorance and considering themselves intelligent and enlightened, the senseless people go round and round, following crooked courses, just like the blind led by the blind." I. ii. 5.
"The intelligent man gives up happiness and sorrow by developing concentration of mind on the Self and thereby meditating on the old Deity who is inscrutable, lodged inaccessibly, located in the intellect, and seated in the midst of misery." I. ii. 12.
"The intelligent Self is neither born nor does It die. It did not originate from anything, nor did anything originate from it. It is birth less, eternal, undecaying, and ancient. It is not injured even when the it is killed." I. ii. 18. "The Self that is subtler than the subtle, and greater than the great, is lodged in the heart of every creature. A desire less man sees that glory of the Self through the serenity of the organs and thereby he becomes free from sorrow." I. ii. 20.
And on and on goes the dialogue between the two great knower of the Truth.
It is wonderful fact that if the Teacher and the taught are of highest qualifications, it is a matter of minutes to enter the state of samadhi. As the Teacher explains so does the disciple experiences the Truths spoken.
Nachiketa gets established into the highest state of bliss where 'knowledge of Brahman becomes a fact of direct experience'. A similar situation can be cited when Arjuna experiences the Cosmic form of Sri Krishna when the Lord tells him about the Bhagavad Gita