The book chronicles the fictitious travels and speeches of Zarathustra. Zarathustra's namesake was the founder of Zoroastrianism, usually known in English as Zoroaster (). Nietzsche is clearly portraying a "new" or "different" Zarathustra, one who turns traditional morality on its head. He goes on to characterize "what the name of Zarathustra means in my mouth, the mouth of the first immoralist:"
Zarathustra has a simple characterisation and plot, narrated sporadically throughout the text. It possesses a unique experimental style, one that is, for instance, evident in newly invented "dithyrambs" narrated or sung by Zarathustra. Likewise, the separate Dithyrambs of Dionysus was written in autumn 1888, and printed with the full volume in 1892, as the corollaries of Zarathustra's "abundance".
Some speculate that Nietzsche intended to write about final acts of creation and destruction brought about by Zarathustra. However, the book lacks a finale to match that description; its actual ending focuses more on Zarathustra recognizing that his legacy is beginning to perpetuate, and consequently choosing to leave the higher men to their own devices in carrying his legacy forth.
Zarathustra also contains the famous dictum "God is dead", which had appeared earlier in The Gay Science. In his autobiographical work Ecce Homo, Nietzsche states that the book's underlying concept is discussed within "the penultimate section of the fourth book" of 'The Gay Science' (Ecce Homo, Kaufmann). It is the eternal recurrence of the same events.
This concept first occurred to Nietzsche while he was walking in Switzerland through the woods along the lake of Silvaplana (close to Surlej); he was inspired by the sight of a gigantic, towering, pyramidal rock. Before Zarathustra, Nietzsche had mentioned the concept in the fourth book of The Gay Science (e.g., sect. 341); this was the first public proclamation of the notion by him. Apart from its salient presence in Zarathustra, it is also echoed throughout Nietzsche's work. At any rate, it is by Zarathustra's transfiguration that he embraces eternity, that he at last ascertains "the supreme will to power". This inspiration finds its expression with Zarathustra's roundelay, featured twice in the book, once near the story's close:
Another singular feature of Zarathustra, first presented in the prologue, is the designation of human beings as a transition between apes and the "Übermensch" (in English, either the "overman" or "superman"; or, superhuman or overhuman. English translators Thomas Common and R. J. Hollingdale use superman, while Kaufmann uses overman, and Parkes uses overhuman. Martin has opted to leave the nearly universally understood term as Übermensch in his new translation). The Übermensch is one of the many interconnecting, interdependent themes of the story, and is represented through several different metaphors. Examples include: the lightning that is portended by the silence and raindrops of a travelling storm cloud; or the sun's rise and culmination at its midday zenith; or a man traversing a rope stationed above an abyss, moving away from his uncultivated animality and towards the Übermensch.
The symbol of the Übermensch also alludes to Nietzsche's notions of "self-mastery", "self-cultivation", "self-direction", and "self-overcoming". Expounding these concepts, Zarathustra declares:
The book embodies a number of innovative poetical and rhetorical methods of expression. It serves as a parallel and supplement to the various philosophical ideas present in Nietzsche's body of work. He has, however, said that "among my writings my Zarathustra stands to my mind by itself" (Ecce Homo, Preface, sec. 4, Kaufmann). Emphasizing its centrality and its status as his magnum opus, Nietzsche stated that:
Since many of the book's ideas are also present in his other works, Zarathustra is seen to have served as a precursor to his later philosophical thought. With the book, Nietzsche embraced a distinct aesthetic assiduity. He later reformulated many of his ideas, in Beyond Good and Evil and various other writings that he composed thereafter. He continued to emphasize his philosophical concerns; generally, his intention was to show an alternative to repressive moral codes and to avert "nihilism" in all of its varied forms.
Other aspects of Thus Spoke Zarathustra relate to Nietzsche's proposed "Transvaluation of All Values". This incomplete project began with The Antichrist.