Emily Blunt's Rachel is drunk. She drinks possibly partly because her ex-husband is horny. While Rachel was married to him he got horny for a Swedish-looking blonde then dumped Rachel and married the Swedish-looking blonde. They have a baby, and a young nanny, who is also blonde, but not really Swedish-looking.
The nanny is also horny. Very horny. We see flashbacks of her in therapy sessions, talking about being horny. This makes the therapist horny.
The horny nanny and her husband live next door to Rachel's ex-husband and his new family.
Rachel travels on the train a lot. Usually while drunk.
The train goes past Rachel's old home, where her horny ex-husband now lives with his Swedish-looking blonde wife. And…