The story is in the grimoire cards. Surprisingly, the destiny app is faster than bungie's site. Anyway, the main threat in the world of Destiny is the darkness, which is the Hive. The Hive took the moon and made it their base. With that threat known, the Last City assembled the greatest team of Guardians to take out Crota, but failed, and Eris was the only one to come back after having her light taken by Crota.
So what do a bunch of pissed off Guardians have better to do? They took out Crota and now his dad is pissed and he's coming for you, nigga!
My only complaint here is that TTK is too expensive for a DLC, especially if the base game is the same price and has more content.