What has gone on before...
After recovering from the Dai Kioshin induced heart disease, Goku is stronger than ever. It quickly becomes apparent, however, that even Dr. Gero's destruction will not bring about an end to the chaos, and there is greater evil yet in the world. This takes form in the being known as Cell, who is desperate to reach his own completion. His search is severely hampered by an out-break of new androids, the origin of which is uncertain, and motives unclear. In the ensuing battle, Goku becomes something new, becoming part-android.
Though Cell reached his Perfect form, he unwittingly triggered Goku's transformation to Super Saiyan 2 and, consequently, his own destruction. With Cell out of the way, the world returns to what is more or less peace once again. The lone surviving android is 18, who is left alone and completely uncertain as to what to do.
Without any other hold on life, 18 joins Goku in his wasteland home. It is evident, however, that dark forces will accumulate again, and that the stakes in the overall battle have been raised.
Characters
Goku: continuing to become more powerful, Goku has gone to the next level. But he's also changing, in ways that he cannot comprehend. Though he continues to protect Earth, he also struggles with his evolution.
Piccolo: playing little role in the recent conflict, Piccolo none-the-less continues to play a role in the greater conflict. As Dai Kioshin grows more desperate, his role will become more important.
Dai Kioshin: angry at Goku for defying him, Dai Kioshin is ready to take the gloves off. If he can help it, Goku will accomplish nothing.
Android 18: now the only surviving android. Despite her coldness, she is wise enough to take Goku's offer at life. Being rejected by the rest of society, she lives in the same house as Goku. 18 is very confused about what she intends with her future.