Plot

Years in the future, a group of space-born humans fight to remove the Earth of alien invaders.

Humanity is on the brink of extinction in the future. An object called The Traveler that possess great knowledge and technology came shortly after the present day and humanity was in their golden age expanding throughout the solar system. The golden age lasted for hundreds of years. Until The Traveler's greatest enemy called The Darkness came to the solar system and wiped out most of humanity. The Traveller has been silent ever since. They built a city under the The Traveler as a last ditch resort to save the human race. The ones who guard the city are called Guardians who possess powers gifted from The Traveler. Many Alien races have come as "minions" of the darkness .and they will stop at nothing. If you fail, humanity will be destroyed.—Anonymous

When the human race lands a manned spacecraft on, Mars, they discover a magical being known as the Traveler. Over time, the Traveler causes a golden age in humanity's technology and way of life, letting them travel through space with ease, expanding human lifespan, and helping humans colonize other planets. But the Traveler's worst enemy has been hunting it for years, and now, it has found it, thus ending the golden age and nearly entirely wiping out the human race. In a desperate attempt to save mankind, the Traveler posts itself above Earth, where humans builds the Last City. It then takes a protective measure by sending out "ghosts" to awaken long-dead Warriors that wield light as their greatest weapon, in an attempt to save the Traveler and restore light to the world. One of these warriors, known as guardians, is you.

The following in-game quotes will tell you all the you need to know about the storyline, and all you will ever know about the storyline even if you play the game:

THE SPEAKER: You must have no end of questions, Guardian. In its dying breath, The Traveler created the Ghosts to seek out those who can wield its Light as a weapon--Guardians--to protect us and do what the Traveler itself no longer can. GUARDIAN: What happened to it? THE SPEAKER: I could tell you of the great battle centuries ago, how the Traveler was crippled. I could tell you of the power of The Darkness, its ancient enemy. There are many tales told throughout the City to frighten children. Lately, those tales have stopped. Now... the children are frightened anyway. The Darkness is coming back. We will not survive it this time. GHOST: Its armies surround us. The Fallen are just the beginning. GUARDIAN: What can I do? THE SPEAKER: You must push back the Darkness. Guardians are fighting on Earth and beyond. Join them. Your Ghost will guide you. I only hope he chose wisely.

Here is also another quote which might tell you some more:

Exo-Stranger: I dont even have time to tell you why i dont even have time.

This is seriously what Bungie gives to tell you the story of the game.