Just keep one guy in mind; Desmond, a former bartender who comes from a very long and odd line of assassins. Through the use of the Animus, we find out that he can experience the lives of said assassin family line simply by using the machine. Except the people who had possession of the machine was Abstergo (Templars), they kidnap Desmond, and after his escape continue to chase him until they use a new form of data extraction (all you need is the blood of anyone from the same family line).
Through his exploration with the Animus he eventually finds out about God-like beings called the "Isu" , who through experimentation made humans and the pieces of Eden (which were used to leash all humans into submission). However as is always the case, Interbreeding with Isu and Humans created a Hybrid race which was resistant to all pieces of Eden (like Diablo and the Nephlem). These few remaining "free" humans started a war with said Isu and all shyt broke loose. Unfortunately for both races, the looming doom of a major extinction level disaster quelled nearly both sides.
This is where Desmond comes in, he meets the Isu through ancient holograms, and finds out that another ELD is due (Solar Flare) in 2012. Inevitably he finds a way to stop the disaster...but it came at a price, sacrificing himself. This act also releases Juno, a not so human friendly Isu.
Pretty much the quickest wrap up. Desmond sacrifices himself, Earth saved...Juno is released.
