Of the entirely of Halo lore?
....
Uh.. Precursors created Ancient Humanity, Forerunner (Halo building dudes), everything basically. A long long time ago. (In this Galaxy, not so far away)
Forerunners killed/drove off most/all of the Precursors because they were kinda dicks.
What Precursors remained eventually became the Flood Parasite, attacked ancient humanity. Ancient humanity fought off the Flood but also got involved fighting the Forerunners as well. Forerunners eventually won due to the humans fighting on two fronts. The leader of the Forerunner military was a being named the Didact, who hated ancient humanity with a passion.
The Flood then attacked the Forerunners and over a long while, eventually got to the point where they had almost destroyed the Forerunners completely. The Didact did what he could to win the war, but it looked to be impossible.
As a last ditch effort, the Forerunners developed the Halo array to kill all sentient life in the galaxy, thus depriving the Flood of food and effectively neutralizing it. This would kill all non-Flood as well (Humans, Forerunner, etc). To counteract this, a Forerunner named "The Librarian" developed a way to 'reseed' life to the galaxy after the Halo firing, so Humans/etc could live again. Some Forerunner deemed the humans the next "caretakers of The Mantle" since the Forerunners chose not to reseed themselves, and those who did not perish removed themselves from the galaxy to parts unknown.
Fast forward to 2500's and modern/future humanity (United Nations Space Command, UNSC) is out there among the stars, dealing with planetary insurrection/drama. The Spartan-II project is created to fight this insurrection, effectively creating Super Soldiers, stronger, faster, better in every way. These super soldiers were kidnapped at age 6 and trained and augmented for years to become the best fighting force in the galaxy*. Spartan-117 aka "The Master Chief" is one of these super soldiers.
Humanity also has developed seriously awesome A.I. beings, who are created based on a brain scan of a formerly alive human. They have a typical lifespan of about 7 years. One of the pinnacles of A.I. at the time is a construct named "Cortana".
Eventually, the USNC encounter the Covenant.. a group of hostile alien beings hellbent on the destruction of humanity at all costs. The Covenant believe the ancient Forerunners are their gods, and the relics they left behind are guides to help the Covenant on a "Great Journey" which is culminated in the firing of the Halo Array (again).
Lots of fighting ensues, the Spartan-II's win lots of wars, but the Covenant is too strong. Eventually the Halo rings are rediscovered by the UNSC and Covenant, the Flood (Dormant aboard the Halo rings for study) are released, culminating in a new plague sweeping the galaxy. Spartan-117 "Master Chief" is assigned the A.I. "Cortana" to help him with his missions, they become best buds.
Due to their fanatism, the Covenant want to fire the Halo Array again to go on the "Great Journey", but the UNSC does not want to, as they know it will just kill everyone again, human/Covenant/Flood alike.
Eventually, all of this comes to a head on "The Ark" an extra-galactic creation where the Halo Array is fired from. The Flood, the Covenant and the UNSC are all squaring off for the future of everyone. Spartan-117 manages to get an unfinished Halo ring to fire on The Ark, presumably killing all the flood, while the UNSC ships escape to Earth.
The Covenant is effectively destroyed, with the various alien races fighting each other. Some of the Covenant forces form a truce with the UNSC.
Spartan-117 "Master Chief" did not make it back to Earth, as his ship was left floating in the void of space for years. Cortana is left along while Master Chief goes into hibernation.
Eventually he was guided+ to a Forerunner construct planet named "Requiem". On Requiem, the Master Chief finds out that the Didact (see above) has been encased in hibernation for thousands of years. Once he is awoken, he immediately goes back to hating humanity and working towards their enslavement.
More fighting between the Didacts forces, the remaining enemy Covenant forces and the UNSC.
The Didact eventually reaches Earth and begins to enslave (by digitizing) humanity, the Master Chief and Cortana eventually stop him, destroy his ship and escape. However, Cortana used all her remaining power to get the Master Chief off the ship and she appears to be destroyed.
Master Chief returns to Earth, and continues to do missions for the UNSC, going wherever he is needed.
And I think that brings us mostly to where we are now. Lots of details were skipped but I tried to make this brief.. and failed!
* debatable I guess.
+ Allegedly
Edit: I just realized maybe you were asking for Cliff notes on "The Flood" novel. In that case, it's just a retelling of the Halo: Combat Evolved game, with more detail given to parts not shown in the game (What were the Marines doing? What were the Covenant doing? Etc).