On April 19th, 1949, HMS Amethyst, a frigate of the Royal Navy, left Shanghai for Woosung, and passage up the great Yangtse river to the Chinese capital. On a peaceful mission, the Amethyst was unexpectedly attacked by the Chinese People's Liberation Army on the north bank of the river. Taken by surprise, the amethyst ran aground. Many of the crew were killed. With rescue attempts blocked by the Chinese communist army, a skeleton crew refloat the ship, but, closely watched and covered by shore batteries, they are prisoners. For a long time the situation remains in deadlock, until finally, lieutenant-commander John S. Kerans realizes he must make a dangerous decision trust their captors to agree a deal, or run the gauntlet of communist batteries, down 140 miles of hostile territory.