Mr Mason is a rich Muggle builder who Vernon Dursley hoped would order a large number of drills from his firm, Grunnings. Mr Mason and his wife were invited to Vernon's house for dinner, where they met his wife Petunia and their son, Dudley.
The evening started off well, however Vernon's nephew Harry Potter (who Mr Mason knew nothing about) unintentionally caused a disturbance when a house-elf called Dobby made a lot of noise in his room and destroyed the magnificent pudding Petunia had prepared. Some ice-cream saved dessert, however when an owl flew straight into the living room dropped a letter on his wife's head. She ran screaming from the house, and Mr Mason explained that his wife was terrified of birds and wanted to know if they were playing some sort of joke, before leaving himself.