The premise of the programme is set around a slimming club in the Headingley district of Leeds. The club is run by the formidable Carol, who fruitlessly tries to persuade the members of the group to follow the 'Super Slimmers' diet. The characters who attend the club are from varied backgrounds and of varied weight. Whilst some plots continue throughout all four series, each episode focuses on one particular character.
Lauren Harris - Gaynor Faye (series 1-4) - a Jewish actress of average weight. Lauren thinks she is a disappointment to her parents, not being married and struggling to get much acting work. In series 1, she has been dating a non-Jewish fitness instructor called Gareth but that ends when her younger sister announces her engagement to Ben. In series 2, she starts dating a vicar named Paul and later marries him, despite her parents' clear disapproval. Her father also disapproves of her making a living by holding Ann Summers style parties.
Kelly Chadwick - Ruth Jones (series 1-4) - Kelly works at her father's fish and chip shop and marries Kevin in the first series. Feeling broody at the sight of her much younger brother, Russell, she and Kevin begin trying for a baby almost immediately and go for IVF when unsuccessful. Kelly conceives naturally and gives birth to her son in series 3.
Rebecca Patterson - Lisa Riley (series 2-4) - Rebecca is a neurotic garden centre worker, who is scarred from a life of bullying. Aided by her boyfriend, she sees that everyone has slight body issues.
Betty Simpson - Alison Steadman (series 1-4) - mother of Kelly and wife of Douglas. Betty becomes pregnant while in her forties in the first series and discovers her adopted son in series two. She also meets her first boyfriend and father of the son she gave up for adoption - Douglas isn't happy about her seeing this man and so, to save her marriage, she agrees to stop seeing him. Lauren's boyfriend, Paul, helps her track down her son and they begin to build a relationship, despite jealousy from Simon's adoptive parents and Kelly.
Jamie Rymer - James Corden (series 1-4) - Jamie is fifteen and bullied at school in the first series; while at school, he looks after his depressed mother until school bullies push him to attempt suicide. Following this, his reasons for eating so much are revealed and his mother tries not to lean on him so much. Jamie inherits money from his Gran on his seventeenth birthday, only to be defrauded by a local spiv who poses as his friend. In series three, Jamie, now 20, meets a girl on the internet who lives in a ground floor council flat in Burmantofts. The flat is constantly under attack from local thugs and a jealous ex-husband. In series four, the young woman is killed by a hit and run driver, leaving Jamie to bring up her young child. Jamie is advised to hand the child over to her father but on seeing his violent behaviour, goes to court for an order giving him residency.