Imprisoned in the dungeon of the Kali Temple, a stronghold of the Thuggee cult, a British officer is whipped then threatened with being thrown into a pit full of snakes if he doesn't tell them what they want to know. The fear of all those harmless snakes coiling and wiggling and sticking their tongues out is too much for the guy and he gives in.
Later, after delivering a great speach about dying for India, his country and his faith, the head Thuggee guru heroically throws himself into the pit to kill himself. We don't get to see him poisoned to death by a pit full of harmless movie snakes, we only see the looks of terror on the British soldiers' faces as he falls in.
The snakes are mostly gopher snakes, with a California kingsnake and a boa constrictor and some sort of snakes with fake cobra hoods attached to their necks. The snakes are pulled around by strings tied to them. The strings are visible in shots of the snake pit if you stop the movie and look closely, which, of cours, few people 1939 had the ability to do.