All Upanishads are written in Sanskrit, and the commentary by Acharya Shankara is taken as the most authentic. Upanishads are based on the dialogue between a realized soul acting as the Teacher, Rishi, and a sincere seeker of Truth who approaches Him as a disciple.
In Katha Upanishad the teacher is Yama - The Death Himself - and the student is a young Nachiketa in his teens. This Upanishad is one of the most popular Upanishads for its simplicity and clarity in making the subject matter regarding the highest truth easily comprehensible. It consists altogether of 120 verses.
This summary is based on the English exposition of Katha Upanishad by Swami Ghambhirananda of Ramakrishna Order, which in turn takes Shankara's commentary as its basis. I am not a scholar, nor do I know Sanskrit. I accept the Truths of Upanishads because I know Sri Ramakrishna had objectified - personified - Upanishadic Truths in his life. Later Swami Vivekananda, his foremost disciple, decided to make all these truths broadcast all over the globe for the benefit of humanity.
As the story goes, Vajashrava, Nachiketa's father, decided to acquire fruits of sacrificial ritual -Viswajit Yagna- in which the performer had to give away all his precious possessions. Cows were designated as valuable and special possessions in those very ancient days, and hence Vajashrava decided to donate all his cows to Brahmins.
Nachiketa was in his teens, and he 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, cows 'who had given their milk for the last time and who were not capable of bearing calves'.
This 'worldly cleverness' of his father produced unfathomable change in the heart of this young boy in whose heart now entered Shraddha - Faith. Nachiketa, in order to dissuade his father from engaging in further mean acts, asked, "O father, 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 insistent. He repeated the question thrice when the irritated father said, "O. K. O Nachiketa, I give you to Death."
Thus ordained, 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 praised sincerity of Nachiketa to wait for him, 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.
"O Death, of the three boons you have offered me, I ask for the first to the effect that my father may become freed from anxiety about me and he may recognize me and talk to me when freed by you,"
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. There is a dialogue between Yama and the boy about the primordial Fire and sacrificial rituals to attain to heavenly life. Yama tells him about the methods and ways of performing these Yagnas 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 transcended both hunger and thirst, and crossed over sorrow, one rejoices in the heavenly world.
The Third Boon
Then comes the main subject matter of this Upanishad. As regards his third boon, Nachiketa wants to know:
"This doubt that arises, consequent on the death of a man - some saying 'It (The SELF) exists', and others saying 'It (The SELF) does not exist'. I would like to know this, under your instruction, O Death, what is the Truth."
Nachiketa had 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 subtle questions of immense intricacies for Death was not sure whether Nachiketa was qualified to receive this knowledge for which only an occasional and rare aspires. Yama says to the boy, "O Nachiketa, ask for health, life, riches, jewels, and enjoyment. Ask for lasting kingdom and armies, ask for anything in this world or of heaven, I will grant you all this as your third boon, but do not force me to go into the secrets of life and death. Do not insist for ultimate knowledge."
But Nachiketa argues that all worldly treasures and heavenly pleasures come to an end sooner or later. If not day after, after hundred years. These are not permanent means of enjoyment. He insists to get the ultimate knowledge of Self, 'for, O Death, you have promised me the third boon'.
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.
And as Death goes on elaborating the subtlety and nuances of means and methods to achieve that transcendental state, consciousness of Nachiketa also is getting established in that altered state to experience those Truths.
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 highest state of bliss where 'knowledge of Brahman becomes a fact of direct experience'. The same thing can be seen when Arjuna experiences the cosmic form of Sri Krishna when the Lord is telling him the Gita!