Bella isn't evil. But even people with good intentions can end up doing bad tings. Especially when they meet people with the power to influence them to do almost anything, like John Wilkes Booth - the most charismatic and famous actor of his time.
So, when Booth sets his sights on Bella, an assistant seamstress to Mary Todd Lincoln, to help with his plot agains President Lincoln, he is able to persuade her to betray both her beliefs and the boy she loves. Deep down, Bella still knows right from wrong, and she ultimately puts her own safety on the line to save the President. But it's too late for Booth to turn back, and he won't let anyone stand in his way - even if it means hurting Bella.
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