This Upanishad appears in the Katha branch of Black Yajurveda.
This is in the form of story which mainly narrates the dialogues of Nachiketa with Lord of death, Yama.
In short the story goes like this:
Once upon a time there was a person called Vajashrava. He performs the Yajna(Vishvajit Yajna) with the desire to obtain the fruit. He has a son called Nachiketa. Though a child, Nachiketa could observe that his father while donating the cows during the yajna, is offerring old cows whose milk has dried up and cannot produce the cattle. He was pained by his father's attitude and resolved to save his father by the self-sacrifice from the ill fruits of yajna which he may receive due to this .
Hence he goes to his father and asking that as he is donating everything available in the house, whom does he intends to donate his son i.e. Nachiketa himself? Nachiketa being still a child, initially father(Vajashrava) ignores him. But when he keeps pasterring and insists for the answer, with anger he tells his son that he is going to give him away to death.
Usually donation is to obtain the favours from the person whom it is given(because the yajna was performed to get the benificial fruit). Hence Nachiketa starts thinking as to what must be his father desirous to obtain from Lord of death? Though the father repents later while thinking of the consequences of his utterance, Nachiketa consoles his father and proceeds to meet the lord of death Yama.
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To be continued......