it starts around one hundred and fifty years before Twilight.
Maya is a vampire that physically she is at the lowest end of the scale regarding speed and strength, but she has a very unique ability, to learned and mimic other vampires ability up to a certain extent. Aro saw the possibility he had with Maya and started teaching other vampires ability to her, but it always came with a price. She stopped learning and Aro thought she needed to be punished, so he exiled her, remove her hands which is the source of how her ability works and appointed a soldier to guard her.
William is the soldier from the Volturi that Aro ordered to guard Maya during her exile. Opposite to Maya he is very strong (as Emmet) and fast (as Edward) which would compensate Maya´s lack of skills and would be able to protect her considering how incapacitated she would be without any ability to use or even any hands to protect herself.
Aro considered him one of his best choice since William likes to follow orders and believes in what the Volturi are doing, but just as an extra safety measure he got Chelsea to create a bond so strong that distance and time would not dissolve, the only thing that Aro did not considered is William falling in love for Maya.
This is just a small explanation of the main characters of the story, but the story moves first as how the relationship between Maya and William develops during those 150 years (which is not very long), afterwards how Aro tries to bring Maya up to action once again, after those pesky Cullens start to get on the way and acquiring to much power and after how the Cullens together with the shape shifter, Denalis, William and Maya start working against Aro´s newest plan.