Harold (Peter Sellers) leaves his fiance Joyce (Joyce Van Patten) at the altar, having come to be fascinated by the hippie lifestyle Nancy (Leigh Taylor-Young) and his own brother embody. He embraces this new life with Nancy completely, but finds he can't have a committed relationship - hardly even a moment alone - with her. In fact, he hallucinates all the people from his old life amongst the revelers at a party Nancy's friends have started in his apartment, realizing that he only traded one kind of status-seeking for another.
For a time, he reverts back to his old life to the point that he once more is at the altar with Joyce. But again, he leaves her there, realizing that he doesn't know *what* he's now running out into the world looking for, but whatever it may be, it's "something beautiful" that will bring him true happiness.