I now understand all this.
I understand all this with Adam and his sister Eileen and Eileens present fiancee Russ and and Mike and Mona. All it is is this. Adam is a pilot and finally has a Sunday off so he can be with Mona at his place all to himself, finally without his pilot career getting in the way. However Adam gets a surprise visit from his sister Eileen because she is unsure of herself about her present relationship with Russ and needs his help. In Eileens and Russ's relationship, Russ hasn't shown any love to Eileen in a very long time and it left her confused, that was before she took the train from Albany to New York City.
However Adam had plans to make love and have you know what(the birds and the bees) in his apartment with Mona but Eileen showed up so Adam and Mona needed another place to crash at so it couldn't be her place because of her present roomate but when a pilot friend of Adams says his place is available, Eileen shows up saying that Mr. Drysdale has an emergency on call flight for him. So he and Mona have to go to the airport, but when Morgan shows up last minute confusion that screws up Adams and Monas relationship happens when Mona boards the plane. Then Mona is angry in Pittsburgh but Adam doesn't give up and after all those crazy confusions the only way Adam could save his relationship was to marry her. He would have had her on that flight to Denver if he hadn't have slipped on that stupid banana peel.
Now as far as Eileen goes she had to get on a Fifth Avenue Bus to go to the Rockefeller Skating Rink to let her brother Adam know that Mr. Drysdale was looking for him for an emergency flight but Morgan would have shown up if that 3 year old son of his hadn't have locked himself in the bathroom.
As far as Mike goes she met him on the bus on her way to see Adam and the two coats collided into each other. Then they both just happened to go to have coffee at the restuarant in Rockefeller Center then second time they met on the bus he decides to take her boat rowing in Central Park and after that relax at Adams apartment.
Plus the reason she wanted Mike to make love to her despite the factor that was engaged to Russ with problems in it was because she was starved for love because lately until Russ shows up later he was neglecting Eileen. However long time after Eileen realizes despite the factor that they have only known each other for one day falling in love just happened to both of them and lately before Eileens trip down to NY to visit her brother Adam, Russ wasn't doing much in that months time.
The point of this movie is that love and relationships between many two parties are all different and lots of them have confusions.