CREATE TABLE table_3961 ("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 in the series had a production code of bdf405?
SELECT MIN("Series #") FROM table_3961 WHERE "Production code" = 'BDF405'
SELECT MIN("Series #") FROM "table_3961" WHERE "Production code" = 'BDF405'