At the beginning of the original Naruto series, all we knew was that Naruto’s parents died in the attack of a Nine-tailed fox. It was slightly hinted that the Fourth Hokage, who died saving the village after sealing the Nine-tailed fox into a newly born infant, could be related to Naruto. Later on, Naruto Shippuden Naruto finally met his parents in the Pain arc and the War arc in Naruto.
When Naruto was born, a mysterious masked man- Tobi (Obito Uchiha), attacked Konoha. He took advantage of the situation when the seal was weak during the time of labour in Jinchuriki.
However, Minato succeeded in stopping the attack and sealed away the half of Nine tails in his son Naruto thinking he could master it someday. But, at the same time, he sealed the other half in himself by splitting the Biju into two with Reaper’s death seal.
When the seal was about to complete, Kurama (Kyuubi) tried to kill the infant Naruto. Both Kushina and Minato jumped in front of the fox’s pawn to save their child.
They both bid goodbyes to Naruto and sealed some of their chakra inside him, thinking they could someday meet their grownup Naruto. Well, this scene was the most heartbreaking to watch in the entire series.