[3] On 19 April 1949, the Royal Navy frigate HMS Amethyst sails up the Yangtze River on her way to Nanjing, the Chinese capital, to deliver supplies to the British Embassy.

Suddenly, without warning, People's Liberation Army (PLA) shore batteries open fire and, after a heavy engagement, Amethyst lies grounded in the mud and badly damaged.

An attempt to evacuate the wounded is only partially successful – the officers of the Amethyst become aware that two seamen have been captured by the PLA and are being held at a nearby military hospital.

Lieutenant-Commander John Kerans (Richard Todd), assistant naval attaché in nearby Nanjing, is ordered to go to the beleaguered ship and take command.

When the shore batteries finally notice the frigate escaping downriver, the merchantman receives the brunt of the PLA artillery and catches fire, while Amethyst presses on at top speed.

Encountering an obstruction in the river in the form of several sunken ships, and having no proper equipment for charting a safe course, Kerans uses both intuition and luck to slip through before then reaching the guns and searchlights of Woosung.

After she is inevitably spotted, the Amethyst is forced into a lengthy fight with the PLA batteries as she flees with all guns blazing, heading for the mouth of the river just beyond.

The film then ends with scrolling text reciting verbatim the message sent the very same day from King George VI, commending the crew for their "courage, skill and determination".

As the Amethyst's main engines were no longer operational, her sister Black Swan-class sloop HMS Magpie stood in for shots of the ship moving and firing of her guns.

Teazer is depicted firing her guns broadside and turning at speed in the narrow confines of the Stour estuary as Consort attempts to get a towing line to Amethyst under heavy gunfire.