The timeline is during the Buu Saga. Cell was never defeated, neither was Frieza, but it all started about 200 Years ago.
Planet Vegeta had been overrun by Frieza's men. He only had one chance to stop Frieza and save his planet. He and 10 other elite Saiyans stormed Frieza's ship. Although Frieza killed King Vegeta and those men, this is not the case of this story. Frieza was caught off guard and beaten to a pulp. He was thrown off of his ship and sent back to his planet. After all of his henchmen were kill or exiled King Vegeta took back his Planet and defended it for the next 200 Years. Frieza devastated stayed on his planet and watched his empire crumble. All 72 of his controlled planets other than Planet Frieza were freed.
200 Years later he had rebuilt his army and was ready for round 2. The Saiyans still on Vegeta have been flourishing. Goku though was sent to Earth. The Androids were set free recently though chaos and havoc is not on the menu. Their purpose now is to scout the Earth and report to Gero on what was happening on land. Cell was also set loose, though still at a bug stage he is not able to transform until he reaches his potential.
Babidi under the radar has been creating his demon Buu, waiting for the perfect moment to set it free. While the Galaxy seems to be safe for now, chaos is soon to come.
It is always calm before the storm.
Dragon Ball Z: Conflicts Return