The eight remaining best cooks in the competition head to the biggest naval base in Western Europe to embark on a special mission to serve lunch to members of the Royal Navy on board one of its longest serving ships, anti-submarine frigate HMS Northumberland. The semi-finalists face the enormous challenge of running their own kitchens in two specially constructed catering tents. Split into two teams of four, each team have to create two main courses and a dessert to feed 110 members of the ships company. After judging their food, John and Gregg pick the weakest team to return to the MasterChef kitchen, where they must cook-off against each other in a special Invention Test. One contestant goes home at the end.