The Siddhartha of the book and movie is NOT Gautama Siddhartha, the Buddha. They merely share the same name, though the title character crosses path with the Buddha and studies with him for a time. But they soon diverge when the younger man sees the flaws in seeking knowledge from others and vows to "listen to no teacher forevermore." Instead, he embarks on his own experiential journey, passing through asceticism, materialism, sexual desire, and the inevitable emptiness that follows all pleasures and pains of mortality.
In the end, Siddhartha returns to the Ganges River, where he is truly enlightened by the ferryman, for whom he apprentices, then replaces on the river. Near the end of his life, Kamala, the courtesan he shared his passions with, comes to the river crossing, following the Buddha, and is there bit by an asp. She dies, but not before bequeathing to Siddhartha a teenaged son he never knew he had. The son rejects Siddhartha and leaves, and for the first time Siddhartha knows loss, which is the root of all pain, according to the eight-fold path.
From this nadir of existence, Siddhartha eventually recovers, and finally meets his childhood friend Govinda, who has been a follower of the Buddha, but now comes to Siddhartha for further englightenment. "Look to the river," said Siddhartha. "Everything changes, but everything returns as well."
This abandonment of desire in the understanding of the cyclical nature of life is the ultimate truth of Buddhism, and Siddhartha finds release from the cares of the world as he recognizes that only in wanting nothing can one be truly free.
Sven Nykvist, Bergman's famous cinematographer, paints each scene in loving colors and shadow and the water metaphor is continued throughout, somewhat clumsily. The movie races along, in order to include all Siddhartha's stages of life, and so the result is a catalog of yearnings and the disppointment of achievement, but the bigger lesson shines through: we are spiritual beings, seeking knowledge and wisdom. We must not let life interfere with our quest.
It's a good thing Siddhartha narrates what's going on with him because not one single character or emotional metamorphosis makes any sense. Things just seem to happen...people react to things...stuff stops happening...and then Shashi Kapoor tells us what he is thinking in the hopes that it might cover the actors' inabilities to convey any feeling and the filmmakers' overall ineptitude.