ASSASSINS CREED: BROTHERHOOD STORY LINE?

Ive played assassins creed Brotherhood and beat it. This is the only assassins creed ive played. and im totally confused about the story. Whyd Desmoind kill Lucy? Who is the ghost girl screaming at him? Whered the "Apple" come into play? and whats the apple? Whats the story? Why are they doing this? Why does the team want the apple? Whens the next AC coming out and will it be a continuation of AC:Brotherhood? Just please give me a decent and long explanation on what the story is im soo confused. Thanks points to best answer! :)