The Mersey is one of Britain's most magnificent rivers, running 70 miles from Stockport to the Irish Sea. This documentary reveals how the waterway has been at the heart of British shipbuilding for more than 180 years, from the first aircraft carrier HMS Ark Royal to the latest, HMS Prince of Wales. It has also been important for business, with 40 per cent of all world trade coming through Liverpool's docks in the 19th century, and played a key role in the Allied victory during the Second World War.