Spoilers ahead…
Karnan is a humanist retelling of parts of the Mahabharata and the legend of King Arthur and his sword, Excalibur. Both are fused together in a superb myth-making scene at the beginning of the film: Karnan (Dhanush) slices a fish and becomes the custodian of the community's sword and wins over Draupadi's (Rajisha Vijayan) heart. But Mari Selvaraj fashions mythology into something that represents Podiyankulam and the oppressed community that's confined to it.
Arthur's Excalibur gives him the right to rule Britain; Karnan is merely the custodian of a collective property, the sword doesn't give any power or right. In fact, throughout the film we see him sharing it with others. He won the sword in a competition and there isn't anything predestined about it. More importantly, King Arthur needed the right to rule while Karnan's people needed the right to survive. Superficially modeled on the Anglo-Saxon legend, Karnan is in an inverted image of King Arthur; Karnan has no destiny save for the one he chooses.
When Karnan bisects the fish and wins the sword, it looks like he's thrusting it into the clouds and opening a small crack of light. This is not a weapon from heaven, but one that opens it.
In the Mahabharata, Karna is spurned by Draupadi during a competition to choose her suitor on account of his birth. In Karnan, Draupadi is in love with Karnan and hopes that he would win the sword. She judges based on actions, not birth. She would have judged Karnan above even King Arthur.
The counterpoint to Karnan's sword are the several headless figures in the film. The titles appear over a headless Bodhisattva, it's head commonly lost due to vandalism. But it's also believed to represent a personality without identity. There's a painting of Che Guevara without a face. Even the blood-thirsty police officer, Kannabiran (Nataraj Subramanian), who is killed by Karnan is reduced to a chalk drawing of the body and a splatter of red for head in the end.
In addition to this grand, mythical framing, Karnan also has minor, fascinating narratives that add layers of interpretation to what seems to be the usual mythical journey of a hero.
The death of Karnan's sister is just one death in government statistics. Barely ever noticed, the system is now only too eager to also forget her. When both life and death lack dignity, the goddess restores it. Just as Jesus saves humanity in the Bible by giving them his own flesh as bread, the local goddess of Podiyankulam gives the statistic without identity a face—her own. She becomes the head of those who are deceased, as a way of making sure their suffering isn't forgotten. She's the head of those forsaken by the headless Bodhisattva.
And in fact, the chain of events leading to the smashing of the bus and the rebellion in Podiyankulam begins with a dream. Collective memories gather force with a momentum individual brains cannot, and Karnan's sister — now with the goddess's head — appears in her father's dream, pointing to hidden treasure in their house. They do find treasure —a few coins.
But this scene foreshadows Karnan's destiny: he will dig out his people's treasure—their freedom. Their goddess has buried it under them and seems to represent the insistent thrust forward of Podiyankulam's past. She's not merely a deity to whom they supplicate for favors. She's a living presence of their lives—and deaths.
In 'Uttradheenga Yeppov' children wear masks of the goddess as the village prepares for a faceoff with the police. So far, village elders had prevented children from knowing or reacting to the community's past, so they could move on and get government jobs. But the goddess won't let them, the peoples' own subconscious won't, and the children dance as goddesses to reclaim their identity.
By using the device of a dream, Mari Selvaraj connects the religious and psychological. A lofty god in the sky is replaced by one that is a kind of collective conscience. It's as if injustices of the past can be forgotten but never erased permanently from the mind. The native religion of Podiyankulam appears to be an organic expression of their lifestyle. Their gods are as humble and practical as the people.
You could read Karnan as the story of a donkey learning to ride a horse. Early on in the film, we are introduced to a donkey with its front legs tied (to prevent escape). It hasn't given up, though. Once in a while, it lifts up its front legs but can't free itself.
We also meet a horse belonging to a little boy, the first and only horse in Podiyankulam. He treasures it, but never rides it. It's practically a donkey for him.
When Karnan frees the donkey it gracefully gallops like a horse (though the horse in the film hasn't) and climbs a hill to stand next to his sister with the goddess's face. Karnan has allegorically freed his own mind through the donkey; but can a donkey ever become a horse, even with untied feet?
Later in the film, when Karnan mounts a horse (like the donkey climbed the hill) towards his destiny, you see how the 'donkey', a thankless beast of burden, now has the skill to commandeer a 'superior' animal after it's shackles are removed. What's interesting is that Karnan is not the first person in his village to mount the horse.
It's the little boy who keeps the horse who figures it out. This makes sense in a film that often puts the collective before the personal. In 'Thattaan Thattaan' the words 'jeyichidu kannu' might be addressed to Karnan, but the image we see as we hear the words is that of the hopping donkey.
Though the donkey and horse bear most of the metaphorical load in the film, we get frequent episodes with or intercuts to various other animals. The mere fact that Podiyankulam has an elephant angers a caste group. We get visuals of eagles swooping to steal chickens with Karnan arguing that there's no point in begging eagles to be kind. That's the first scene of the film and also its whole in a nutshell.
And it's not just that animals are used as ornamental metaphors. Pariyerum Perumal was set in a law college which provided an ironic backdrop for a film on caste discrimination. Karnan, especially in the first half, feels like a story of people set in a world of animals. It's as if Mari Selvaraj wanted us to think of natural law and not constitutional law as the solution to the problems depicted in Karnan.
For example, in a scene where the village is gathered in the commons to decide on a response to a specific instance of everyday injustice — no bus stops in Podiyankulam — you hear the bleating of goats which stops once the group agrees to take risky action. The goats seem as much a part of the conversation as the people who can often sound as scared. Members of the oppressing caste have their meetings in spacious and well-appointed homes, totally cut off from nature.
A fight between Dhanush and members of the opposing group happens in the middle of a herd of buffaloes. Their intergenerational conflict — implacable as a buffalo — forces them to fight amidst the suffocation. Crows, snails, cats, eagles, they all play a role in the film as a reflection of the world view of people who live in close contact with animals and hence, likely to think of the world in terms of them.
In the second half of the film, once the State has taken over the conflict, we don't see as many animals. Animals recede as the human war intensifies and breaks both human and natural laws.