The Cell Series
After the vicious battle against Freeza, our heroes enter the age of Cell...

After years of anguish from a defeat that took place decades ago, Dr. Gero continues to plot revenge against the character who destroyed Red Ribbon and ruined him- Gokou. Android #19 is the first of a series of creations to destroy Gokou. Along with the scheme, Dr. Gero requests Android #19 to make him into a cyborg so that he would become stronger, and this bears witness of the transformation of Dr. Gero to Android #20. The Z Warriors meet with Androids #19 and #20 and the two sides engage in battle. Gokou leaves the battle while Vegeta faces Android #19. The android is destroyed and Dr. Gero is shattered. Despite the grave defeat, Dr. Gero's cybernetic programming enables him to self-repair, so that he may live to fight again.
Android #20 then works on developing a better android model. Meanwhile, he unleashes two of his other creations, Android #17 and Android #18. Androids #17 and #18 get annoyed of Dr. Gero. As a result, Android #17 kills Dr. Gero. But, the doctor had just finished creating Android #21, who is still developing in a giant test tube. Before he dies, he issues the final command to his new android to feast on the rebellionous androids. Android #17 and Android #18 leave the lab and unleash Android #16. But as it turns out, Android #16 is a peaceful android. (This can be shown through his love towards the wild animals.) As time passes, the Z Warriors become aware of the new androids as they battle again and again but always reach a stalemate. However, the Z Warriors are still not match for the androids because the androids are robots, and therefore do not tire in a long battle while the Z Warriors eventually run out of energy. Gokou suffers a heart attack in one battle and retreats. Trunks, who was sent back in time by future Bulma with medicine to heal Gokou's heart so that Gokou would live to kill the androids and thus change the future, gives Gokou the medicine. Gokou is saved, but the Saiyans still do not continue to fight the androids. Instead, they train in a chamber to get stronger. Meanwhile, Piccolo fuses with Kami, his half father, to become stronger (His story is told in the Dragonball series.) Dende, a Namek child, replaces Kami as guardian of the dragonballs.
Androids #16, #17, and #18 go to seek Gokou to destroy him. Piccolo finds the androids first and begins to fight them. Back in Dr. Gero's lab, Cell has finished developing and breaks out of the giant "test tube", promising to fulfill his creator's wish.
Cell flies to find Androids #17 and #18. On his way devours the souls of the unlucky civilians he passes to become stronger. Finally, he meets with the other androids. To our heroes' surprise, they learn that Androids #17 and #18 know nothing about Cell. Cell and Android #17 threaten to kill each other. Android #17 heads out to fight him. But, he does not stand up to Cell and is easily defeated. This is when our heroes learn the great powers of Cell. Android #18 is now worried, but Android #16 flies out to fight Cell. To everybody's amazement, Android #16 is able to stand up to Cell. However, he is still destroyed. Cell now heads for his goal and devours Android #17 with his tail. Cell now changes into Cell--form 2. With the power of Android #17 combined to his, he becomes stronger than ever. Then, he heads over to devour Android #18. Android #18 tries to escape, but she is still eventually devoured. Cell now changes into Cell form 3, and becomes "Perfect Cell" Cell, after devouring the souls of so many people, is no longer an android and becomes organic.
After Cell is gone, Bulma collects the body parts of Android #16 and repairs him. Android 16 is re-programmed to fight on the Z Warriors' side. Our heroes now head out to fight Cell, and Android #16 is once again destroyed. Cell grows strong enough and reproduces. From his body come 7 Cell Juniors, miniature blue duplicates of Cell. With the death of Android 16, an enraged teenage Gohan powers up and reaches Super Saiyan level 2. Gohan then destroys all the Cell Juniors and fights Cell. In a following battle, Cell is kicked and Android #18 is spit back out. (She is unconscious.) Cell turns back into form 2 and is defeated. But, he activates his self destruction mode so that when he dies, the world will blow up with him. To save the Earth, Gokou teleports Cell out of Earth and into Kaiou's planet, where Cell blows up. Thus, Gokou, Kaiou, Bubbles, and Gregory die as well.
The Z Warriors mourn the loss of Gokou, but cheer at the victory over Cell at the same time. A fireball flies out of nowhere and pierces through Trunks. Upon Trunks' death, Cell, in form 3, appears. He tells that when he blew up, a brain cell survived. From the cell, other cells multiplied and he was once again formed. Cell happily laughs as he tells that he is amazed he is able to rebuild himself into form 3! Then, Cell threatens to destroy the earth. Now it is up to Gohan to try to save our planet. With one arm (the other injured when he went to let a fireball meant for Vegeta hit him), Gohan prepares a fireball. He fires at Cell and Cell fires back. A stalemate is meant, but with Kaiou's help, Gokou is able to talk to Gohan and encourages him. Meanwhile, Klilyn protects the unconscious Android #18 and Yamcha, Tenshin-han, and Vegeta attack Cell in hope that this will Gohan distract Cell. With the help of the now dead Kaiou, Gokou, also dead, is able to communicate with Gohan and encourages him to use all his power to defeat Cell. Gohan overpowers Cell and Cell is disintegrated by Gohan's fireball. (But, Sataan takes credit for Cell's death because of his lies to the reporters.)
At the end, the warriors gather the dragonballs to bring those killed by Cell back to life. However, Gokou is still dead because he was already brought back to life once and cannot come back again, but tells that he is okay and is now with Kaiou. Android #18 awakens and flees. However, everybody learns here that Klilyn has develped a crush on Android #18! At the end of this series, there is a short memorial about the life of Gokou.

Notes:

Dr. Gero is angry at Gokou because Gokou destroyed Red Ribbon, the military corporation Dr. Gero worked for, when Gokou was just a kid. Dr. Gero was a character from Dragonball.

Tao Pai-Pai also makes an appearance in this series. He built a fort and charged people to hide in it. He claimed that it would protect them from Cell. Gohan easily destroyed this building and Tao Pai-Pai fled away once again.

Cell is really from the future. In Future Trunks's timeline, Cell killed Trunks and stole his time machine to travel back into the timeline we know of.