Kind-hearted, fun-loving, and utterly without purpose, Arthur spends every day in the heedless pursuit of amusement. But when his unpredictable public image threatens the staid reputation of the family foundation, Bach Worldwide, he is given an ultimatum: marry the beautiful but decidedly unlovable Susan Johnson (Jennifer Garner), an ambitious corporate exec who can keep him in line, or say goodbye to his billion-dollar inheritance and the only way of life he knows. It's a deal Arthur would be inclined to take...if he hadn't just fallen for Naomi (Greta Gerwig), a New York City tour guide who shares his idealism and spontaneity. All he needs to do is stand up for what he wants. But at what cost? With some unconventional help from Hobson (Helen Mirren)--the one person who always believed he could do anything--Arthur will take the most expensive risk of his life and learn what it means to become a man.