The format was original for its time: a mixture of philosophical meditation,
allegorical fiction, poetry and (for lack of a better word) prophetic rambling.
The protagonist is Zarathustra/ Zoroaster (presumably the prophet of the
ancient Iranian religion, but possibly just someone whose name happens to be
Zarathustra, because this character shows little respect for traditional
moral values, including the very Zoroastrian concept of good and evil fighting
a cosmic battle).
Zarathustra, apparently an hermit who lived on a mountain for ten years, decides to walk back into human society and to spread the gospel of his vision. On the way he meets a saint who praises god and Zarathustra wonders that he hasn't heard the news yet, that god is dead. Alas, the visionary hermit ends up among the crowd assembled to enjoy the shows of a circus, not the most receptive or intelligence of crowds. Now the challenge is to spread the gospel among the crowd that is watching an acrobat on a tightrope. He tells them of the Superman, a wiser evolution of man, and scorns the "last men" who "invented happiness". The crowd loudly invokes the "last man", not the Superman. A jester causes the acrobat to fall and die and then threatens Zarathustra as he walks away carrying the dead body. On the way out Zarathustra sees an eagle and serpent, and then goes "under". Then he delivers 22 speeches to his followers.
In contrast to Platonic and Christian ideals, that view this world as a mere wildly imperfect shadow of the real one, Nietzsche seems to love this world, or, at least, thinks that we should learn to love it and stick with it. It is emblematic that Zarathustra leaves the cave with knowledge of the truth, not with knowledge that the truth is impossible to achieve.
The "last man" is the man who has become mere routine: society's moral values help shape the "last man" by imposing the same laws of what is good on everybody. But creating a society in which only pleasure exists is a recipe for creating absolute unhappiness. Pleasure is relative to pain, and so cannot exist without pain. The perfect society preached by the Utilitarians ("the greatest happiness for the greatest number of people") is anathema to Nietzsche.
Nietzsche views the human race as a stage in the evolution from animals to "Superman", a rope (a bridge?) between animals and the Superman. Nietzsche does not provide a scientific definition of this creature, in particular we don't know if it refers to a new biological species, to a machine or simply to the equivalent of the Buddhist enlightenment. Sentences such as "Man is something that shall be overcome" do not quite clarify whether this Superman will be the work of humans capable of transcending the limitations of the current generations or the work of biological evolution. Sentences such as "All beings so far have created something beyond themselves" are probably just misrepresentation of Darwin's theory of evolution (that was a brand new thing in Nietzsche's time): obviously it is not the worm that creates another species but the combination of variation and selection over millions of years.
After returning to his mountain cave for a few years, Zarathustra has a dream that he interprets as a message that his teaching is being distorted. He rejoins his followers and tells them of the "will to power". Schopenhauer thought that everything that exists has and is driven by "will", whether a stone or a scientific genius. That is the will to live. But Nietzsche thinks that there is a more powerful force at work, particularly in human nature: the "will to power". Zarathustra is torn by doubts about his philosophy, as he realizes that he is being too rational like Apollo (the contrary impression that a reader like me gets) and not enough irrational like Dionysus. Trying to regain that balance, he finds solace in singing and dancing. A dream spoils disturbs his peace of mind, and he meets someone that reminds him that life seems to have no meaning. Zarathustra doubts whether his "Superman" is yet another pointless project and whether he himself is truly a revolutionary or simply yet another intellectual in the old cultural tradition. A series of encounters further deepens his state of uncertainty.
Zarathustra wanders alone for a while as he heads for the highest mountain top presumably a metaphor to mean that he is about to become the Superman. After confronting a dwarf who makes fun of his effort, Zarathustra explains the fact that everything in the universe is doomed to repeat itself sooner or later, the eternal recurrence of everything (this is a few years before Poincare's theorem), and the dwars actually agrees with him: "Time itself is a circle." Zarathustra seems to understand the implications: that the Superman has happened before, and that has followed by a return to ordinary men, and so forth. "The center is everywhere". In between, Zarathustra has time to look down on ordinary men who call cowardice "virtue" and mediocrity "moderation". He spits on the city full of cowards. He thinks that the best are those who want to rule and therefore they will rule. He returns to his cave. He learns to sing and to dance. And he sings a love song to eternity.
If you think that the first three parts have been boring, the fourth one will surprise you becaues it is even more boring.
An old Zarathustra meets a number of people on his mountain: the same sage who told him that life is meaningless, two kings who are disgusted by the mediocre people of their kingdoms, the conscientious man of the spirit (who is trying to attrack leeches to his veins), a trembling old man who turns out to be a magician pretending to be an ascetic, the last pope after god died (god died of too much compassion), the ugliest man who turns out to be god's murderer (he killed god because god was too compassionate), the voluntary beggar who has given away all his fortune to be among the poor but finds the poor as mediocre as the rich, and his own shadow. He invites all of them to his cave and finds them there in the evening. He is disappointed that they are nowhere near the "high man" that he was hoping for. They share a "last supper" (a parody of Jesus' last supper). Zarathustra tells them how he started his preaching in the wrong place, the market where the crowd hailed equality and god and refused to understand that the Superman has to come. When Zarathustra leaves the cave, the magician impersonates the devil and tries to seduce the others, but the conscientious man of the spirit realizes what he is up to and counters him with the power of science. Zarathustra leaves them after another pretentious and somewhat offensive speech. Suddenly his guests are all happy and noisy. Zarathustra walks back into the cave and finds them praying to a donkey. The ugliest man has resurrected the god that he himself had killed. Zarathustra, of course, is angry and instead encourages them to worship Dionysos. Zarathustra feels pity for them even if he thinks that pity is a sin.
By all of this, Nietzsche criticizes (scorns) the Christian way of life that undermines real life on this real planet in the vain hope of a better afterlife that will never take place.
A lion appears and inspires him to resume his journey and his preaching.
This is also in conflict with Schopenhauer's teaching that we should renounce the material world (a` la Buddha) whereas Nietzsche thinks that our material world is good at it is and we should just enjoy this life without hoping in a better afterlife. Regrets and remorses are bad signs because they indicate that one has not accepted this life.
In general the book is annoyingly vague and pretentious while containing precious little meaning. We never learn what the Superman is or does. We never learn what is the thing that died and that Nietzsche calls "god" (my feeling is that he is simply referring to traditional morality and the traditional way of life in the Christian world). We never learn what kind of morality should come next.
I have rarely been so utterly bored reading a book. I think that Nietzsche had nothing to say and, in fact, didn't say it.