It may be that I've seen more movies with Dev Patel over the past few years than movies with any other actor. How would I check that? Hm.
The movie is based on true events: back in 2008, ten Islamist terrorists smuggled themselves into Mumbai and proceeded to murder people. When it was all over, four days later, 166 people had been killed. (Also nine out of the ten terrorists.)
It's much like an action movie, except there's no Bruce Willis or Harrison Ford to crawl around in ductwork to save the innocent civilians. It's just innocent civilians, woefully unarmed against the automatic rifles and grenades of the villains. And (worse) Mumbai had no SWAT team on hand to effectively respond to the invasion; the local cops are brave, but out-weaponed.
The attack was on a number of Mumbai locales, but the movie concentrates on the Taj Hotel, a glamorous leftover from the days of the Raj. Dev Patel is a waiter in the hotel's gourmet restaurant, presided over by a strict head chef. Ironically, he's nearly sent home for forgetting to wear his nice shoes to work; his boss takes pity on him, lends him his own shoes. And so he's on hand for all the carnage.
The movie (understandably) focuses on a few guests: a rich couple, their nanny, their infant son, a Russian ex-Spetznaz officer, a cowardly young man who puts his girlfriend in danger. Not all of them make it to the end of the movie.
It's pretty harrowing, the violence is unglamorized and brutal. The simple heroism of everyday people is the primary theme.