The week's best chefs return for the quarter-finals and this is where the battle really begins. Their first challenge is an Invention Test - but this one has a twist. The quarter-finalists must create an exceptional plate of food from the kitchen offcuts and trimmings. Gone are the array of quality ingredients they are used to cooking with. Instead, the judges ask them to make their dishes from kitchen scraps, including bones, trimmings, fish skins, over-ripe fruits, cheese and stale bread. It's a daunting challenge and the price is high - after judging their dishes, Marcus and Monica decide who is worthy of continuing in the competition. Next, the chefs face the terrifying task of showcasing their skill to three of the country's toughest restaurant critics. The critics expect two exceptional courses and the chefs only have one hour and 15 minutes to produce an outstanding menu - good enough to secure their MasterChef future. Sitting in judgment of their culinary abilities are critics Charles Campion, William Sitwell and Jay Rayner. Only the two most skilful chefs can make it through to the semi-finals and continue the fight for the coveted title of Professional MasterChef 2014.