When 00:21 is the total footage (mm:ss) what is the serial?
CREATE TABLE "table1_1889619_5" (
  "doctor" text,
  "season" text,
  "story_no" text,
  "serial" text,
  "number_of_episodes" text,
  "total_footage_remaining_from_missing_episodes_mm_ss" text,
  "missing_episodes_with_recovered_footage" text,
  "country_territory" text,
  "source" text,
  "format" text,
  "total_footage_mm_ss" text
);
SELECT "serial" FROM "table1_1889619_5" WHERE "total_footage_mm_ss"='00:21';