Catherine 'Cathy' Walcott and Catherine 'Cat' Walcott only discovered one another existed after their dad dropped dead. Having finally come to terms with the fact they are sisters, they are now reeling from the news that they also have a brother, Callum - and that Cathy just snogged him. As Cathy struggles with the ethics of fancying a sibling, Cat is determined to welcome Callum to the family, while Callum isn't sure what family means any more.

Cat's mum, Marilyn, is furious to learn she wasn't Colin's only mistress and sets about tracking down his other secret lover, Angela, starting a chain of events that nobody expected. Meanwhile, Cathy's mum, Tess, decides to have some fun with her late husband's legacy and embarks on an ill-advised luxury spending spree.

Cat & Cathy go to an adult maths GCSE class where the sexy teacher causes ructions. Tess takes a job in the worst place possible, and Angela resorts to kidnap to win back Peter.