The story of Gaav concerns a small, isolated village in the Iranian “desert” that has only a single cow. The owner of the cow, Hassan, has no children and obsessively devotes all his attention and affection to his cow, which is perhaps the basis for his importance in the community. One day while Hassan is attending to some affair in another village, the cow is found dead in its stall. The villagers are grief-stricken by the calamity, but their greatest concern is the effect that this will have on Hassan when he learns of it. So they conspire to tell a story to Hassan that the cow has run away and they are still looking for it. In the meantime they bury the cow in an abandoned village well.
When Hassan returns, he is stunned by the disappearance of the cow and quickly shows signs of withdrawal into a world of disbelief and madness. He doesn’t believe the far-fetched story that he is told about the cow running away, and he ultimately drifts into the state of believing that he, himself, is the cow. He retires to the stall and mournfully limits himself to eating hay and occasionally mooing like a cow. The villagers are nonplussed by the situation and try to come up with ways of bringing Hassan back to life. All of their efforts come to nothing, however, and they eventually decide that the only thing left for them to do is to harness him to a rope and drag him to a hospital in a larger city. The arduous task of dragging the bound-up Hassan across the barren landscape during a rainstorm (one of the most memorable scenes in the film) leads the leader of the group, Eslam, in frustration, to whip Hassan like a pack animal. Hassan’s destruction as a man is complete. The subsequent events, when Hassan breaks away from his captors and falls down a steep slope and dies, seem inevitable.
Iran's Ayatollah Khomeini was reported to have admired this film. This in turn was reported to have been the saving grace that allowed Iranian cinema to continue without becoming completely banned after the Iranian Revolution.

en.wikipedia.org/wiki/The_Cow_(film)

Dariush Mehrjui discusses "The Cow"

The Cow