He first was mentioned by Tia when she said that she last saw Mia working for Big D at his night club. Then at the night club, Mater asks Laverne if she's seen Mia anywhere, and Laverne says she saw her a few days ago with Big D, but with the smell of salt, like the ocean.
Then came the part when Big D actually appeared. When the Crane took Mater's boxes off, Mater got surrounded by Big D's henchmen. Then Big D came. Mater said that he will tell the DA. Then Big D says that he won't tell the DA anything. Then when he saw Lightning McQueen and some other pollice come, he said, "Oh, no! McQueen!" Then he tried to escape, but Mater moved the crane with his tow hook and made it drop a crate of tires on him, which were revealed as counterfiets. Then Lightning and the pollice arrested Big D.