Shopaholic and Sister chronicles the travails of a shopaholic extraordinaire, Becky Bloomwood. She gets married to Luke Brandon and they travel around the world for their ten-month long honeymoon. During this trip, Becky picks up various mementos, unbeknownst to her husband. She makes expensive purchases including a leather belt, a handbag, wooden giraffes and kilim rugs. They are keen to get back to London after their long stay away from home.
When they return, Becky finds her parents behaving strangely. She is also perturbed to find that her best friend has found another friend. She eventually finds out that she has a half-sister, Jess, through her father before he got married to her mother. When Becky meets Jess, she is heartbroken to find that she is nothing like herself. Jess is frugal, doesn't care for fashion and would prefer caves to shoe stores. She is the complete opposite of Becky.
Becky and Jess have a falling-out before long. Meanwhile, the purchases that she had made during her honeymoon arrive at her home and her husband is furious to see the price tags of these purchases, some of which he specifically forbade her from buying.