NEW DELHI — The most popular movie in India this summer is about a toilet. It nearly causes a divorce. It makes a father slap his adult son. It splits a village in half. But, ultimately, it’s about a romance.

More than just an ode to the commode, the film, “Toilet, a Love Story,” speaks to one of India’s most serious public health concerns. Toilets are a big issue in India these days: There aren’t enough of them for the country’s 1.3 billion people, and the national government is embarking on the biggest toilet-building campaign in the nation’s history.

India’s prime minister, Narendra Modi, troubled by how many Indians still relieve themselves in the open, has vowed to build a staggering 100 million new toilets.

All across the country, new latrines are going up, sometimes so fast they are not connected to anything, creating toilets to nowhere that are so fly-ridden and stinky that almost no one will use them.