Superlatively acted story of the explosive nature of secrets. Judi Dench plays a middle-aged teacher who discovers that a co-worker in the school (Cate Blanchett) is having an affair with a 15-year-old student. The film is a lot like Ms. Dench's character: brutally frank, wickedly witty, and caustic as lye. It is also bravely unapologetic in its compassion for these flawed people and an altogether mesmerizing watch.