Season 4 has the Android/Cell Saga and here are some things that happen:
-Mirai (future) Trunks comes from the time machine.
-Vegeta goes Super Saiyan in the room of Spirit and Time which is a room where 1 year there is 1 Earth day.
-Dr. Gero creates Androids and Dr. Gero is Android 20.
-The androids are so strong that Kami and Piccolo fuse to become Kamiccolo.
-When they are still not winning, Goku and Vegeta go to the room of spirit and time to train for a while.
-Everybody discovers Cell in a time machine in an egg and Dr. Gero created Cell.
-Cell fuses with 17 and 18 to become Perfect Cell!
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