The sauce is made from milk, a three-cheese blend, black pepper, and smoked bacon. It's inspired by pasta carbonara, which typically features a sauce made with egg, pancetta (similar to bacon and often substituted with bacon here in the US), cheese, and black pepper.
The price for an order is 390 yen (~$3.47 US).
Other limited-time items currently on the McDonald's Japan menu includes Cinnamon Melts (which were discontinued in the US a while back), Spicy Chicken McNuggets, and a purple sweet potato shake.
Photo via McDonald's.