For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Lilia's bags are all packed up and she's ready to explore Italy. While visiting the small town of Monteriano, Lilia falls head over heels for a dashing young Italian and marries him. Her in-laws back in England, the Stuffy-McUptightersons (oops, the Herritons), are furious and pretty much act as if she doesn't exist anymore. Lilia's happiness is short-lived as her marriage to Gino turns sour, and tragedy strikes when Lilia dies giving birth to her son.
Act II
When news of Lilia's son reaches the town of Sawston, Mrs. Herriton attempts to preserve the family's reputation by sending Philip and Harriet to Italy to gain custody over the child. It's not as though she wants to meet the baby… she just wants everyone in Sawston to think she's the World's Best Grandma. Miss Abbott also goes, to spy on the Herritons and make sure their intentions for the baby are pure. Both Philip and Miss Abbott fail to convince Gino to give up his son, so it's all up to Harriet, who decides that the best plan would be to kidnap the child.
Act III
As Harriet and Philip make their way to the station with the stolen baby, the carriage suddenly crashes and overturns. The baby is tragically killed in the accident. Huh. Who would have thought kidnapping a baby would go wrong? Miss Abbott miraculously manages to keep Gino from killing Philip when he finds out the news. When it comes time to return to England, everyone parts on friendly terms with Gino, because Gino is cool like that. Philip realizes he lurves Miss Abbott, but—dang—Miss Abbott realizes she lurves Gino.