In 1937, WW2 has started for China, with the Japanese invasion. On the southern front, the Chinese army has abandoned a ruined Shanghai, except for one warehouse. This is defiantly defended by one regiment of about 800 troops, where seasoned professionals are reinforced by raw recruits of varying eagerness. The warehouse has 2 attributes: used as a bank vault, it has extra-thick walls; and it is across a small river from the English Concession. The foreign concessions are considered foreign territories by the Japanese, who try to avoid touching them. Thus we have the juxtaposition of a battlefield with a bustling, well-lit area, where crowds hang around the riverside street to watch the action, and the privileged (including foreign press photographers) watch from the balconies.
The movie alternates between the warehouse action, and the events in the concession. In and around the warehouse, the action includes close combat, snipers, and occasional speeches. It is refreshing to see a Chinese action movie without martial arts. While there is blood and bandages, it is not the gorefest some western directors seem to like. There some of the Chinese soldiers are featured, whereas on the Japanese side only one commander had any speaking role. The switching to the concession side gives the audience a break from the action, and keeps the narrative fresh.
This was based on actual historical events, and the movie included a final shot of the warehouse - with a backdrop of modern Shanghai skyscrapers. I read that the movie's opening was delayed, so that scenes of the national government could be cut. This government, or its successors, still govern the rebel island of Taiwan. But for the flag-raising scene, it was still the historically-accurate sun-in-sky national flag that went up.