Spoilers: If you haven't read chapter 1 and 2 yet, read them first.
Also, King Vegeta launched Broly to planetoid Vampa
Vegeta, Raditz, Tarble, Onix, and Tora already went to a mission.
Then Goku was launched to earth.
But Ken waited for Sunday to launch.
On Sunday at 5:00, Ken was preparing to launch.
"Dad, be safe," he said to Bardock.
"I will" He answered.
When the pod launched, Ken clearly saw Frieza destroying Planet Vegeta.
Then, Ken raged and became the ssj grade 2.
He also realized that there is a form called Kaioken.
King Kai got that word in his head so Ken tried it out.
"Kaioken!"
And he left the pod (which is destined to go to mars by now) to fight Frieza.
He blasted a yellow ki wave that left Frieza with a bruised look.
Then he finishes off Frieza with ki blasts and combat.
But, He was shot in the chest and got unconscious.
End of Origin