Cassandra Mortmain lives with her family in a tumbledown castle near the coast in England. Their father, James Mortmain, is a selfish, eccentric novelist. Their stepmother, Topaz, a nudist and artist, Rose, the oldest sister,
is very beautiful. Their younger brother is somewhat odd and isolated from people his
age. Stephen Doyle, their ever faithful and gentle caretaker, has lived with the family for years, and has always been in love with Cassandra. In this array of fascinating characters, Cassandra struggles to find her place in the
world and, at the same time, be who she truly is. The entire story is told through Cassandra's diary and her wonderful but naive imagination and outlook on life.
The Mortmains have squeaked by on residuals from James's one and only book. But, when a letter arrives with news that the book is out of print, they seem doomed to be evicted from the castle. When their landlords, two brothers from America, come to the castle on a rainy night after having car trouble, it seems that Lady Luck is on their side. Frustrated by her family's poverty and seeing an opportunity, Rose determines to catch Simon, the older brother, even though she does not love him. Cassandra helps her sister, hoping her sister will fall in love with Simon as well as have his wealth help their family from its dire straits. Neil, the younger brother, quickly sees through Rose's scheme and they take an almost instant dislike to each other. The arrival of Simon's family from London will throw the Mortmain family's unstable subsistence into a whirlwind of emotions, complex realities, secrets revealed, and affairs of the heart that they could never have dreamed of.
Over time Cassandra falls in love with Simon. But her love is to be kept hidden because he is Rose's. Meanwhile, Cassandra has to deal with Stephen's unwanted attention. He is gorgeous and very caring of Cassandra and her family, and has been in unrequited love with Cassandra for many years. But their relationship is tarnished when Stephen tries to make love to her, divulging his passionate love and obsession for her. When he realizes that Cassandra loves Simon and will never love him back, Stephen leaves for London. He becomes a model and lives with the much older female photographer who seduces him, in an attractive studio/apartment.
Cassandra's world becomes even more entangled one night when she is alone at the castle. Simon comes for a visit and spends the evening with her, eventually dancing with her at his family's home in a beautiful room, and kissing her. Cassandra is astonished at her response to Simon, and begins to despise her sister and her own forbidden desire for Simon.
Cassandra receives a letter from Rose and goes to London, where Rose is staying with the Cotton family as they prepare for Rose and Simon's wedding. Cassandra quickly finds that not all is well with the couple, and upset to see Rose treating Simon with disdain. Later that night Rose admits she does not love Simon, but will marry him anyway so that their family will be taken care of. She and Cassandra have a terrible fight when Rose realizes that Cassandra has fallen in love with Simon. Cassandra runs away into the night, realizes she has no money, and phones Stephen to help her get back home.
The next morning Cassandra finds out that Rose and Neil ran away with each other later after her fight with Rose, finally admitting the love they denied for the past several months. Simon is heartbroken but unable to stop loving Rose much to Cassandra's pain and anger.
There is a side story of Topaz becoming jealous of James and fearing his is having an affair with Simon and Neil's mother. Cassandra and her brother, determined to make their father write again, imprison him in a tower on their property, supplying him with his typewriter, paper, food, etc. At first James refuses to actually write anything again but finally breaks through his writer's block. Topaz returns from London and is horrified to find that James has been held int the tower. But she is overjoyed to see him writing again and they fall in love all over again.
James's new book becomes a hit and he returns to London to begin book tours. Stephen decides being a model is not for him, and he leaves with a friend of Cassandra's.
Cassandra leaves her hope of ever being with Simon, and resolves to waiting for fate to bring her someone who will be her true love. The ending is wonderful, a great finish to an adult look at the developing of Cassandra spirit and mind, leaving the veiwer with the realisation that all her innocence is shed and that she can now face the world without rose tinted glasses. She has hope and she knows that she will indeed live to become a talented and happy woman. After all she will love in the future to come.