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The fourth programme sees Simon cross Northern India under the dark storm clouds of the annual Monsoon, one of the great climactic events of the Tropics.

From the coast of Gujarat to the teeming metropolis of Kolkata (Calcutta), this journey takes him on across areas of India rarely visited by tourists.

In the east he visits the "Little Rann", a unique desert environment and home to the last viable population of the Indian Wild Ass. These shy, beautiful creatures are impossible to keep in captivity, and are under threat from India's vast and ever-expanding human population, another huge issue across much of the Tropics.

Nearby Ahmedebad was the home city of Mahatma Gandhi but, despite his legacy of tolerance, the city has been the scene of vicious clashes between Hindus and Muslims. Simon visits a project that works with traumatised children from both communities.

Passing through the beautiful holy city of Ujjain, the Tropic of Cancer takes Simon to Bhopal, a city notorious as the site of the world's worst industrial accident 25 years ago. Simon is astonished to find the site still contaminated by chemicals and people living nearby complaining of severe health problems.

Just outside Bhopal is the Satpura national park, where Simon investigates the decline of India's tiger population and has a memorable encounter bathing a huge Indian Elephant.

Following the line west takes him through the "Red Corridor" – the scene of a violent Maoist insurgency, where he joins soldiers on the frontline and visits villages caught in the crossfire.

Finally, arriving in Kolkata, Simon ponders a new car so affordable it is expected to sell millions, adding to the city's unbelievable congestion, and sucks the eye out of a fish during a traditional Bengali meal.