The novel commences with the protagonist, Becky Bloomwood, coming to the conclusion of her extravagant ten month-long honeymoon around the world with her husband, Luke. Although the newlyweds have been enjoying themselves, Luke and Becky (nee Bloomwood) decide it is time to return home to England and to follow a normal way of life. Before returning home, the couple embark upon a brief trip to Milan, where Luke is due to meet a potential client. After agreeing with Luke that she will not buy anything in Milan, Becky explores the city. She leaves him unaware that she had secretly planned to buy him a present, a new leather belt, to replace the one she ruined with the hot wax. Becky manages to buy him the belt and was planning to take in some culture next. However, her resolve is weakened when she encounters what she feels as being the chance of a lifetime - the opportunity to possess an Angel handbag, the bag of the season which Becky simply is determined to purchase. Unfortunately, the clerk tells her that there is a waiting list for the bag and has to be put on there. A wealthy businessman, Nathan Temple, helps Becky jump to the head of the waiting list of the handbag, because the owner and him are friends. She shows her gratitude by promising to pay him back somehow. Becky gives Luke a new belt, which he's pleased with believing she has changed.
Back in England, Becky and Luke stop to visit the Bloomwood family and surprise them with their early return. Becky is shocked and dismayed when her parents appear to be hiding something along with the fact they are not impressed with Becky's gifts and don't seem as excited as she had hoped. The next day, Luke and Becky attend the christening of Suze's twins. Unfortunately, Suze has befriended a local woman, Lulu. The two women doesn't get along very well like Suze hoped. During a horse ride together, Lulu finds Becky very immature for her age, while Becky tells her off that she thinks of Lulu as horse-faced and hypocritical. She feels isolated from Suze because Lulu is a mother of four and she feels that they have more in common. Later on, Suze reveals that she was lonely for Becky as she was travelling all over the globe and Lulu was her only source of companionship. However, Becky still believes that Lulu doesn't deserve the benefit of the doubt because she humiliated her earlier, which widens their strained friendship. Luke comforts Becky and tells her that no one expects them back for another month.
Becky's troubles become worse when two trucks full of her souvenirs she has accumulated from her trip arrive. The bills for them are outrageous. With her new job not due to start for several months, Luke orders Becky to sort things out around the house with her spare time. Luke immediately switches back into business-mode, cutting his hair, becoming more efficient and donning expensive suits.
Becky's parents arrive with big news. They explain that they did not want to tell Becky initially, but she has a half-sister from a previous relationship of her father's. Becky is ecstatic - she claims that she has felt a hole in her life and has always wanted a sister. She imagines going shopping together, enjoying girls' nights in, and having a new best friend to replace Suze. Becky is slightly disappointed when she meets Jess: She is studious, thrifty, and a bit standoffish. During a shopping trip, Jess reveals that she doesnt really like shopping and was only going because she wanted to make Becky happy. She decides to try and keep an open mind to the things her sister likes to do, but finds herself missing Suze more when she sees how boring Jess really is. During a trip to a supermarket, Becky wanted to enjoy shoping, but Jess is trying to teach her how to be thrifty. All the while, she can't comprehend why Jess doesn't want to do anything fun and got into a serious fight with her. This was tempoarily broken when a young mother tells both of them to stop fighting as they will be a negative influence on her daughters. When Becky overhears Luke saying that she is hard to live with, Becky is shocked and thinks that her marriage is falling apart. After a fight with Jess, she seeks the help of her parents when she believes her marriage is falling apart. Her mother merely tries to reassure her that it's just a big row that all couples go through and everything will be fine. Becky is temporarily relieved when Luke comes in excited that Brandon C. won over Arcodas.
During the celebration, Becky finds Janet telling an associate of Temple about Luke being well and happy. This got her concerned for a reason as she was trying to stop that meeting for a reason, one that she set up as thanks, but later regretted. Desperate to stop the meeting from happening, Becky tries to convince Temple to let her take him out on the town to one of London's finest bars. He thanks her, but prefers to wait for Luke. Her next attempt is to try and sneak Luke out of Brandon C. After making up some excuse, Becky will return and distract Temple long enough to tell him that he's not interested However, Luke finds out and is livid with Becky when he discovers that she had arranged a meeting for him to see Nathan Temple whom he's not happy about seeing and the fact Becky had gotten an Angel handbag. Both spend a few days arguing as he reveals that he thinks Jess is more helpful than she is because he's been trying to win over Arcodas. In anger and tears, Becky tells Luke off that she had a lot of her own problems to deal with the last few months and he never has time for her. In one moment before leaving, he does show a hint of sympathy and tells her their marriage was not supposed by the way it become. Luke tells Becky that he needs some time away from her so she can think about readjusting her attitude. After he leaves for Cyprus, she reads the lists of problems he has with her and realizes that her marriage has fallen apart.
After a falling-out with Jess and Luke, Becky has a nightmare about transforming into her worst enemy, Alicia, in a fancy pink suit and a horrible sneer. She was laughing and taunting her half-sister for being boring, which crushes her. This however disgusts Luke and Suze who comes to Jess' defense. After that, Becky decides to go to Jess' home in Northern England to learn to be thrifty and make amends with her. Upon arrival to Scully, Jess refuses to see her and doesn't want to help teach her how to be thrifty. When Becky tells her that her marriage fell apart, Jess tells her that she knew it would happen and that Luke would leave her if she continued to behave the way she did. She tells Becky that she will have to learn to save her marriage on her own because it's time for her to grow up. Taking pity on Becky, Jim offers her a place to stay at Edie's Bed and Breakfast as she tries to keep reaching out to Jess. Along the way, Becky learns more about Jess and slowly learns that she had been a spoiled brat all her life. She also starts to understand how appalled her sister and Luke were by her behavior back in London. When Kelly suggested that she gives up on reaching out to her sister by ignoring her, Jim reprimands her by stating that families can't ignore one another. Eventually at an environmental meeting, Jess reveals everything to Becky alone about having a long talk with her aunt, Florence, about her mother's flirtacious past. She was given one name, but her aunt admitted there was more men that may have been Jess' birth father. When she went to Surrey to meet with Graham, she asked him to do a DNA test to see if she was his daughter. Soon Becky realized that because everyone assumed and didn't do a DNA test, she and Jess may not be related and she finally gives up. Before leaving, she makes a heartfelt apology to Jess for her behavior in London. She accepts it and wishes Becky the best of luck saving her marriage to Luke. Robin and the others try to convince her to stay and help out, but she refuses and packs for the return trip to London.
During that time, Jim shows her, Jess' true passion in a cupboard similar to her shoe collection, but it's filled with rocks. Becky realizes her mistake in not giving her half-sister a chance to explain her passion in collecting rocks, calling it boring and a waste of time. Despite their different hobbies, she sees that Jess is the same as her and renews her chances to tell her they are sisters. Unfortunately, she has departed on a mountain climb on a gloomy day, and Becky follows to try to catch up. Unfortunately, Becky is not only wearing the incorrect footwear, but also inappropriately dressed for such to embark upon a hike of that magnitude. She ends up not following the trail and has a nasty fall in dreadful weather. Luckily, Jess has the mental and moral fortitude to watch over Becky. The two girls discover that though their interests are drastically different, they share the same level of passion for their hobbies. Becky realizes she has been spoiled by her parents all her life when she learns more Jess' past in her stepfather's harsh upbringing that made her grow up into the woman she is. While she learned to fend for herself and value money at a young age, Becky's parents always paid for everything and thus she never learned how to do either one. It resulted in her having to rely on others to help her get her out of debt. Becky is finally able to give Jess the silver bean Tiffany necklace that she bought for her as a gift. Jess loves the necklace and Becky finally saw her real weakness aside collecting rocks.
The book concludes with Becky finally relishing in the pleasures being thrifty and frugal. She and Suze are helping Jess organize a protest against a new shopping mall (which turns out to be one of Luke's new clients). Eventually someone finds Becky's Angel handbag and it's returned to her. She refused to accept it, reasoning it was the source of trouble it caused her when both Jess and Luke turned away from her. Instead, she decides to give the bag to Kelly. Becky discovers that she is pregnant.
Nathan Temple: An English businessman who owns Value Motels who helps Becky jump to the head of the waiting list to get an Angel handbag. She pays him back by arranging a meeting with him and Luke in Cyprus. This made Luke very livid with Becky about it, but had to attend because Temple will buy out Daily World and he can't afford to insult him. He ends up liking Temple when he learns about his past and how he got that convicition against him in the first place because he was defending his staff from an abusive drunken guest and things got out of hand.
Jessica 'Jess' Bertram: Becky's older half-sister that her father, Graham, had with Jess' mother, Marguerite, from a past relationship. She was excited because she always wanted a sister and always dreamed about both of them going shopping together, helping her cope with Suze not being her friend and enjoying girls' night in. She was disappointed when she learns that Jess is thrifty, studious and a bit standoffish. Her passion is collecting rocks, which was the one of the issues between her and Becky that lead to a falling out. Jess tells her that she's a spolied brat who lived in a fantasy world. In turn, Becky tells her that she thinks she's boring, miserable, thrifty and has no passions in her life. While stuck in a mountain, during the storm, Jess reveals that she never intended to be rude to Becky, but was unnerved by her because of her parents' harsh upbringing. They reconcile and Becky learns to enjoy being thrifty. She gives Jess the silver Tiffany bean necklace, which she loves and showed Becky, her rock collection. In Mini Shopaholic, it's revealed that Jess and Tom secretly got married in Chile.
Lulu Hetherington: Suze's new best friend and a mother of four children. She and Becky have an immediate dislike towards each other, even though Becky tried to get along with her. She feels isolated when she sees that Suze and Lulu have more in common.
Jim Smith: The owner of Scully Stores who is a friend of Jess' family. He was the one who showed Becky, Jess' rock collection and she immediately regretted what she said about them. Jim also tells her about how harsh both Bill and Marguerite were in her upbringing with her brothers.
Kelly Smith: Jim's teenaged daughter who immediately takes a liking to Becky. She plays a supporting friend to her, smiliar to Suze did in the first three novels. Becky gives her an entire collection of Stila cosmetics.
Robin: The kind-hearted and firm leader of the environmental protest group who Becky befriends. Robin listens to her suggestions for more media attention and a public rally. He and the other protesters likes the idea of it, but Jess is against it. When she tried to get Robin to make Becky leave by pointing out negative things about her, he ignores it and insisted Becky stays. He tells Jess that the environmental meeting is no place to take out personal family problem on her. Robin also tells her that Becky has good ideas that he finds helpful and refreshing over her own monotone, tactical opinions.