After a new computer system called Horizon is rolled out across the country, mysterious financial losses start to appear in village Post Office tills. But when subpostmasters Alan Bates, Jo Hamilton, Noel Thomas and Lee Castleton seek help, the Post Office insists that each of them is the only one having problems. Alan refuses to believe it. And when the Post Office goes on to accuse, harass and even prosecute subpostmasters for the shortfalls, Alan leads the fight back. Their lives, livelihoods and good names destroyed, his little band decide to try to clear their names.