SPOILER ALERT
August 20th, 2010
It is the day of Operation Cobalt, the codename for the military operation that decreed the bombing of major American cities. A submarine is patrolling the waters of the Gulf of Mexico.
Riley (who will reappear in FTWD 6) is an officer on board, recently become a father. After the order to bomb San Francisco, no further communication is received from land.
When a soldier suffering from appendicitis dies during the operation and reanimates, panic erupts on board. Trapped inside the steel vessel, almost everyone succumbs and turns. Only a small group of men, including Riley, manage to survive and reach land near Galveston.
September 12th, 2010
After days spent in isolation on the beach, and realizing that his wife and child are dead, Riley decides to take his own life by shooting himself. But a mysterious man approaches, telling him this is not the end but only the beginning.
That man is Teddy (FTWD 6), and Riley will become his most loyal follower.
P.S.: The dates in the timeline are approximate, but everything happened more or less around those days.