What number episode in the series was watched by 15.89 million U.S. viewers? 
CREATE TABLE "table1_22078906_2" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_airdate" text,
  "u_s_viewers_millions" text
);
SELECT "no_in_series" FROM "table1_22078906_2" WHERE "u_s_viewers_millions"='15.89';