CREATE TABLE table_3958 ("Series #" FLOAT, "Season #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( million ) " TEXT)

What episode number of the series had a production code of bdf409?
SELECT MIN("Series #") FROM table_3958 WHERE "Production code" = 'BDF409'
SELECT MIN("Series #") FROM "table_3958" WHERE "Production code" = 'BDF409'