Dragon Ball Z is the second series of Dragon Ball film and episodes, The story began when Gokou that had married with Chichi has a son, and Goku name it "Son Gohan" the name of his grandfather who took care of Goku's life when Goku was child and even a baby.
Dragon Ball Z Story
The story of Dragon Ball Z began when this family live then there come to them and the world one Saiyan man named "Raditz" Raditz looked for son Goku and claim that he was his brother but the aim of RAditz is to fight Goko and show his Saiyan true power, and finally Raditz met Goku and the fight also finally begins, but it's amazed SonGoku who is the strongest man in the world couldn't beat Raditz and even Raditz beats son Goku because of his big power.
When Goku was almost killed by Radits but it's fortunately Goku was saved by Picollo, and when Goku was alsmost killed by Raditz there is an amazing Incident that Son Gohan, the Goku's first child release his inside power and then save his father, and finally Goku defeated Raditz by sacrifice his life, and Goku was death with Raditz together. But the story has not been over but it's just also begin.