The 'Amazing' Mrs Ros Pritchard is the successful manger of Greengages Supermarket in Eatanswill. When a couple of local politicians make a spectacle of themselves outside her shop, Ros decides to stand for election herself – just to prove that she could do better. Supported (reluctantly) by her husband, Ian, and daughters Emily and Georgina, she begins her campaign, inspiring other women to follow her lead and stand on the same platform. Even the formidable Catherine Walker, the Tory Party Shadow Health Minister, is won over by Ros' charms and defects to her Purple Alliance Party. Meanwhile, Emily manages to break up a wedding and Ian's antics at a work Christmas party nine years ago are plastered all over the papers. Ros' story grips the nation and eight weeks later no one is more surprised than Ros herself when she wins the General Election and becomes the next Prime Minister.

Having been elected as Prime Minister, Ros realises there has been a terrible mistake. She knows nothing about politics, so how can she possibly run the country? Even her husband, Ian, thinks she's proved her point and should now step down. While the Queen tries to persuade Ros to stay, her supporters, Miranda and Catherine, try to put together a Cabinet without knowing anything about its potential members. Ros' first few days in office are a whirlwind of meetings and decisions which leave her family feeling neglected. When Ros has to call the wives of British soldiers who have been murdered in the line of duty, she realises for the first time how tough this job is really going to be.

Ros is on the verge of invoking an Act of Parliament to push through her Reform Bill, which lays out a plan to move Parliament from Westminster to Bradford. The country waits with baited breath but Ros can't seem to make her mind up: she just doesn't quite have the courage yet. Ros struggles to find the words to tell a visiting African President that his family has been assassinated. His dignity and strength inspire her to push on with her Reform Bill. Catherine ends up in bed with Ben, a young policy advisor. She is appalled when he tells her he's in love with her. Emily poses for a lads' mag and soon has her naked image projected onto the Houses of Parliament. She retreats home to Eatanswill with Ian to lick her wounds and buy a flash car with her earnings. Georgina's school life improves when she gets a boyfriend. Ian blurts out his money-laundering secret to Emily.

Twelve months has passed since Ros became Prime Minister. She is now a confident and slick leader who knows how to handle a crisis. That is, until a plane crashes over East London. Racial tension escalates as the country assumes it was a terrorist attack. The Queen finds out that she doesn't have a seat in the plans for the new Parliament building in Bradford. Emily is terrified that her father will end up in prison for money-laundering, or even worse - that her mother will find out. Miranda discovers that Emily has dropped out of university and asks Kitty to offer her a job. Beverley tries to resign when she is photographed stumbling drunkenly out of a bar. Ros chooses to stand by her first-ever Purple recruit, and refuses her resignation.

Ros decides to act for the sake of her children's children and implements a ban on cars every Wednesday to try and reduce gas emissions. The country and Catherine are in uproar but Ros is unrepentant and refuses to back down. A by-election is looming and the Cabinet is worried that anger over the car ban will influence how they vote. Is the tide turning for The Amazing Mrs Pritchard? Miranda is furious with Liz for accepting a loan from Kitty. Catherine feels terrible but can't work out why. Eventually her secretary points out that her symptoms strongly imply she is pregnant. Emily gives up her job with Kitty and comes home to Downing Street. After a row with her mum she lets slip that Ian laundered money 15 years ago.

Ros has finally learnt about Ian's secret money-laundering past. When Miranda tells her that Kitty bribedback benchers to defect during the election, Ros feels that everything is finally slipping away from her. After an emotional chat with Emily, she decides to keep quiet about everything, cover up this mess and get on with her job. That is until Catherine finds out and tells her she has no alternative but to leave Ian or resign. Meanwhile, Catherine is struggling to cope after having a secret abortion. Ben leaves her when he finds out, but she surprises herself by proposing to him. The series ends with Ian summoned to Ros to hear her choice: job or marriage.