'Slans' are genetically superior post-humans, who have gone into hiding to avoid genocide by human normals. I think the novel is informed by awareness of the genocides of the Nazis, and the hero, a slan, is humanist (although not human) and pacifist - though he uses some morally questionable mind control. He befriends a nasty old alcoholic called 'Granny', and protects her life throughout the novel, even when she tries to turn him in to the secret police.
The plot is more complicated than it at first appears, because the slans and the crypto-slans have schemes underway, which the hero - isolated from his kind - uncovers bit by bit. The writing in places rocks along very accessibly, but in places the action was unclear, buried in exposition of background information. At the end I think the whole plot got a bit rushed. It was like the sketch for a more satisfying story. However it's a really quick read, and worth checking out.
This novel is most famous for inspiring the SF slogan 'Fans are Slans', which naively or ironically implies that we SF fans are both persecuted and superior (what a thrilling combination!) but in any case poignantly misunderstood by the cruel masses. And here both fan and slan are metaphors for being an intellectual, a technician, expert, scientist, awkward teenager, and indeed comic book guy or gal.