The movie is not about just a thappad but about not realising it's fault. In the so-called tradition and culture women are always taught to be silent and ignore things.
The protagonist father who love her daughter so much and stands with her, he himself didn't recognise his fault at his early when her wife left her passion for the family, even the tear in her eyes when he says when she was pregnant he never agrue with her, it all tell a different.
We didn't realise the problem until it's ours the causal behaviour of Amu when her maid tells about beating of her by her husband, says it all.
The movie greatly show the pressure on women to ignore everything behind just to hold on family, even the educated, self dependent women bid themselves in these social barriers.