The comical bus that splashes into the river on a family vacation was once a critical piece of World War II military technology. This episode traces the history of the DUKW, known as the Duck, from a logistical problem no landing craft could solve: getting ammunition, food and reinforcements from ships anchored miles offshore directly to the front lines without stopping on a contested beach. It introduces the unlikely design team of yacht designer Rod Stephens Jr., British sailor Dennis Puleston and MIT engineer Frank W. Speir, and explains why the Army and Navy initially rejected a vehicle that fit neither bureaucracy.

The turning point came in 1942 near Provincetown, Massachusetts, when the experimental DUKW drove into a storm with 60-knot winds to rescue seven stranded Coast Guardsmen after conventional boats failed. The episode then breaks down the engineering: the GM factory code behind the name, the paper-thin armor dictated by buoyancy, the bilge pump that kept it afloat with holes in the hull, and the first-ever system for changing tire pressure from inside the cab. It follows the Duck to Sicily, Normandy, the coral reefs of the Pacific, Korea, Antarctica and finally to Wisconsin Dells, where a surplus DUKW launched a tourism industry in 1946.

• What D, U, K and W actually stand for, and why the Duplex Universal Carrier Wheeled explanation is a later myth
• Why a 13,000-pound truck could hit 50 mph on land but only about 5.5 knots in water, and how variable tire pressure let it float over sand and climb coral
• The tragedies alongside the triumphs: overloaded DUKWs capsizing at Omaha Beach and 25 of 26 men lost on Lake Garda in April 1945
• The vehicle’s long afterlife with the British Royal Marines until 2012, the Soviet BAV 485 copy with a rear ramp, and Australian Antarctic expeditions
• How oceanographer Willard Bascom’s crews surfed Pacific waves in DUKWs and how Mel Flath’s Wisconsin Ducks still runs its original routes