Saturn...you know the planet. Actually I think it was Jupiter. Which ever is the biggest planet. Pretty much the run down. The traveler gets a distress call that came from Jupiter (just cheeks it's the bigger planet) and we can't contact them back. So they send the guardians in but not in the ships because of the fear the acid clouds would destroy the ships. So they built halo odst like drop ships for you to go in. Once on the planets service you discover the cobal and another army are fighting it out and other guardians that have disappeared are the ones who sent the call out. But unfortunately you can't contact the traveler to warn them not to send drops ships in. Oh and the clouds of acid rain is a cover up, something the other enemy created so others feared it and couldn't get out. Plus the drop ships are one way anyway. But you have to fight threw to get parts for a ship and on and on. But I don't go into detail about it like you.