We have already read that that vichitrvirya married Ambika and Ambalika. He had two sons and they were Drutharashtra and Pandu.
Drutharashtra being born blind was not eligible to become the king as per the customs of those days. Though he was the eldest. In those it was also the laid down principle that the eldest son of a king would succeed the king.
Pandu becomes the king of Hasthinapura and he being a brave and strong one rules the state with ability.
In those days it was a passion for kings to go on hunting trips one for the game and secondly to protect people from wild animals which used to eat away the villagers cows etc.
Once while on a hunting trip Pandu accidentally kills a deer which was roaming with its wife. In fact the deer was a sage by name Kinnama. The sages in those days because of their penance got power to transform themselves to the forms they wanted and Kinnama and his wife took the form deer.
When Pandu shot at them they get killed but before dying the sage curses Pandu that whenever Pandu meets his wife with love he also would get killed.
Panduraju repents for his action and chooses to go to the forest with his wives Kunthi and Madri. The Pandavas were born in the forest and after the death of Pandu, Kunthi returned to Hasthinapura. The Pandavas grow up in the palace along with the Kauravas.
What this episode of Mahabharata teaches is we must be always vigilant to the surroundings. and will be punished for our acts even if they are committed ignorance. We must always be vigilant.