What happens in dragon ball z episodes? 

The first part: Goku is the strongest warrior on planet Earth. Then a saiyan warrior named  Raditz comes to Earth and claims to be Goku’s elder brother. Goku finds out he is a saiyan. The saiyans are a evil race of warriors that invade other planets and then sell them. Anyways Raditz takes Goku son “Gohan” and says he has to kill a hundred humans before the next day is over or he Will kill his son. Raditz is far more powerful than Goku so piccolo teams up with Goku. They battle it out for a while and at the end Goku holds Raditz down while piccolo charges up a powerful attack called special beam cannon. This attack kills Raditz and it kills Goku. Before Raditz dies he says that more powerful saiyans will come.
 Second part: Now that Goku is dead piccolo takes Gohan and trains him. Mean while Goku is at other world training with the King Kai. Right before the saiyans arrive they wish Goku back with the dragon balls. Goku immediately starts his journey back to Earth. The saiyans get to earth before Goku can. Piccolo and Gohan fight bravely but piccolo dies trying to save Gohan. Goku arrives shortly after this and then Goku kills Nappa the saiyan.  There is only one more saiyan and hes more powerful than Raditz and Nappa, his name is Vegeta.Vegeta  outmatches Goku but Goku learned a move called kaio-ken which is a move king Kai made. It can double, triple, or 10 times power. They end up defeating Vegeta with a spirit bomb. Vegeta flees while Goku is taken to the hospital.
3rd part: This is the part where Frieza comes in. Since piccolo died the dragon balls are gone to. They find that the home planet of piccolo “namek” also has dragon balls. Gohan,krill-in, and bulma take a spaceship to namek. They find out that Vegeta and his ruler with all his henchmen are also at namek looking for the dragon balls. They end up saving a namek kid and eventually they team up with Vegeta. Frieza calls in his special fighters called the Ginyu force. They fight the Ginyu force but are easily beaten. At earth a spaceship is made and he travels to namek. He gets there just and time and defeats the Ginyu force. Goku is hurt by there leader so they put him in a healing tank. They bring piccolo back to life and then fight him without Goku. Frieza transforms several times and he eventually beats krill-in, Gohan,Vegeta, and piccolo. Then Goku is done healing and comes out and fights frieza. Goku is close to frieza but frieza is better. Goku has to summon a spirit bomb to kill frieza. It hurts him but doesn’t kill frieza. Frieza gets mad and kills krill-in. Goku gets mad and turns into a super saiyan. Frieza is no match for Goku and Goku easily wins.

 3rd part: Several enemy’s come and Veg etas son trunks from the future fight a new enemy called androids. At the same time there is also another enemy named cell. Goku, Trunks, Gohan, and Vegeta train and become really powerful. Vegeta easily overpowers cell but cell begs him to let him be his perfect form. When cell does he beats Vegeta. Gohan and new super saiyan comes and kills cell.
 The last enemy is Buu. Buu is extremely powerful. He kills Vegeta and Goku has to fight him .Goku turns super saiyan 3 and then Buu transforms and it takes Vegeta and Goku to finally beat him on the planet of the Ki’s with a large spirit bomb from planet earth.
 All that is left is a world tournament that all the fighters compete in.