Inspired by a true story, a fictional reimagining of a cat named Simon whose heroism and feline charisma won over the crew of a British warship just after World War II. Discovered in the Hong Kong docks in 1948 and smuggled onboard the H.M.S Amethyst by a British sailor who takes pity on the malnourished kitten, Simon quickly acclimates to his new water-borne home. The friendly feline soon established himself as the chief rat-catcher, while also winning the hearts of the entire crew. Then the Amethyst is ordered to sail up the Yangtze River to take over the guarding of the British Embassy, but the ship comes under fire from Communist guns. Tragedy strikes and many of the crew are killed and Simon is among those seriously wounded. Luckily, with the help of the ship's doctor, the brave cat makes a full recovery and is soon spending time with the injured men, purring and keeping their spirits up.

News of Simon's heroics in dangerous wartime seas spreads and he becomes famous worldwide--but it is still a long journey back to England for both the crew and the plucky little cat known as "Able Seacat Simon.".