This is the first part of the Dragon Ball Z series. New characters are introduced since Dragon Ball, such as Gohan (Goku's son), and Goku's past becomes clear as he battles a fighter from another planet.
Vegeta & Nappa Saga
Heeding Radditz' warning about two stronger fighters heading for Earth, the Z warriors train. A long battle with Vegita and Nappa then follows, resulting in the loss of several of the Z warriors.
Freeza & Namek Saga
The remaining Z warriors must travel to Namek in search for Dragon Balls to bring back their fallen comrads. Unfortunately for them, they aren't the only ones seeking the power of the Dragon Balls.
A new character, Trunks, is introduced to the Z warriors, but with bad news for Goku. Trunks' predictions from the future lead the Z warriors into the hardest battle yet with android fighters developed by Dr. Gero.
The Z warriors don't get a break, as a new threat emerges, Dr. Gero's greatest creation, Cell. After single handedly absorbing the androids, Cell goes on to absorb the entire planet of life, with little that can be done to stop him.