Halo 4

Basic ideas (SPOILERS) from just the games:

Halo: Reach: The Covenant discovers the location of Reach, humanity's military hub (where the SPARTAN-II program was conducted), and attacks it. You play as a member of Noble Team, a squad of Spartan-IIIs, defending the doomed planet in a hopeless battle against the Covenant. During the campaign, however, you learn of an object capable of turning the tide of the war in humanity's favor. Every member of Noble Team dies, with the exception of Jun, who is MIA, and Reach is almost completely destroyed by the Covenant.

Halo:Directly after the Fall of Reach, a ship carrying the *last* surviving member of the Spartan-II program named John-117 (or the Master Chief) encounters a ring-like object floating in space. Forced to land on it, you fight the Covenant and learn of a new enemy, called the Flood, which is a parasitic organism capable of transforming hosts into the walking dead. It seems that a previous race, the Forerunner, created Halo to combat and destroy the Flood by decimating every sentient lifeform in the galaxy. Having learned Halo is a weapon of mass destruction, you destroy it and vacate the ring.

Halo 2:This game is unique in the sense that is provides a point of view from the Covenant. Almost immediately after the events of Halo, Master Chief returns to Earth to be decorated for his actions in the previous game. During the ceremony, the Covenant attacks and lands on Earth. After fighting through the city of New Mombasa in Africa, you follow the Covenant to another Halo, where the Flood are already running rampant and a new enemy, called the Gravemind, awaits. After assassinating the Prophet of Regret, you are captured by the Gravemind and teleported to the Covenant's capital ship, High Charity. There you combat the Covenant and the Flood in an attempt to kill the Prophet of Truth, but he escapes and you hitch a ride on his ship to "finish the fight." Also, interspersed throughout the campaign are segements where you play as the Arbiter, an elite condemned for allowing the destruction of Halo, and learn of a civil war between the Elites and the Brutes/Prophets. This schism leads to a great divide in the Covenant. You also learn that there is a network of Halos floating throughout the galaxy, controlled by an intallation dubbed "the Ark."

Halo 3: ODST:You play as an ODST during the aftermath of the New Mombasa slipspace event. Separated from your squad, you travel through the destroyed city at night and discover clues as to your squad's whereabouts. Each clue initates a playable flashback form the perspective of your squadmate. In the end, you rendevouzs with your squad and evacuate the city.

Halo 3:Arriving at Earth, Master Chief and the Arbiter (fighting alongside each other due to the alliance between the humans and the Elites) fight through Africa to secure a portal to the Ark. After the portal is activated, and after fighting some Flood that landed on Earth, you travel through the portal in pursuit of Truth. On the Ark, you race to get to the control room where Truth has access to all of the Halos. After killing Truth, you breach High Charity to rescue Cortana (your AI -- sidekick). You fight the flood and rescue her, but discover that the Halo you destroyed in the first game is almost completely reconstructed. Realizing that a premature detonation would destroy the ring, heavily damage the Ark and thus neautralize the Flood threat, you decide to fire it. After firing it, you race to your ship where you and the Arbiter travel through the portal to Earth, however, the portal closes with the ship halfway in it, leaving the Master Chief stranded in space. The game closes with a hillside memorial to the war and cutscene showing that the Master Chief and Cortana survived the blast and are now floating towards and unknown planet thing.

Halo 4 picks up right where Halo 3 left off.

Hope this is good.