England, 1646. The English civil war is in progress. As battles rage, the Roundhead Army's Leveller faction seeks greater democracy in England. With the Parliamentary victory and execution of the King in 1649, the possibility of further social reform is ended.

St George's Hill, Surrey, April 1649. Gerrard Winstanley and William Everard, dispossessed by the social upheaval of the war, settle a piece of common waste land belonging to Francis Drake, and form a small commune, known as the Diggers, to farm it collectively. Winstanley sets out his Christian philosophy of the earth as a common treasury for all men and his belief that all land and possessions should be held in common. At the nearby house of the local parson, John Platt, his wife reads the writings of Winstanley.

Platt and Winstanley meet, and Platt makes it clear that Winstanley is not welcome in the area. Later, Mrs Platt offers Winstanley a lift in her carriage, and pleads to be allowed to join his community. She reveals that she is pregnant.

Local villagers attack the commune and cart away its members to be imprisoned. They are freed, but reminded that they are guilty of trespass. Soldiers of the Parliamentary Army arrive and are welcomed by Everard. Winstanley and Everard volunteer to meet the Army leader, General Lord Fairfax, to discuss their situation.

At Fairfax's residence, Winstanley and Everard explain their position. Fairfax disapproves of their settlement but is convinced by their commitment to keep the peace. Platt preaches in church against the Diggers, and the villagers again attack the Diggers' settlement and crops. The Diggers rebuild. Winstanley visits Drake and demands freedom for the Diggers to work. While Platt teaches his two children their catechism, his wife gives birth to another boy.

Platt and Fairfax visit St George's Hill. Fairfax inspects the dwellings and livestock, and enquires of the Diggers' professions and morality. He greets their children and tastes their food. Some of the Diggers, including Tom Haydon, have previously fought in Fairfax's army on the Parliamentary side. Fairfax leaves a captain, Gladman, to keep the peace between the Diggers and the local villagers. When Platt arrives home, his wife announces her intention of going to St George's Hill to live with the Diggers.

A group of Ranters, a radical sect, arrive to join the Diggers, but sneer at their religion. Parson Platt and Captain Gladman meet in Platt's garden. Mrs Platt continues to read Winstanley's tract, enraptured. Soldiers attack the commune and kill a boy there. Mrs Platt climbs St George's Hill to discover the murder, scattered pamphlets and a Digger dwelling ablaze.

Winstanley visits Fairfax again to plead for protection. Mrs Platt arrives at the commune to give Winstanley money. Winstanley and other Diggers are summoned to court, where they are refused appearance without an attorney and fined ten pounds for their activities. Cattle from the settlement are seized as payment.

At St George's Hill, the Ranters' leader mocks the Diggers' meal and shows them the food he has stolen. Winstanley upbraids him for his lack of respect for others' property. At harvest time, the Diggers give thanks and reap their meagre crops. Everard returns from travelling with greetings from many more Digger colonies established around the country, and Mrs Platt arrives to join the commune with her child. As the Diggers debate the content of a new tract by Winstanley, a fight breaks out, and a hysterical Ranter is beaten by the Diggers. Mrs Platt flees the commune and returns home to beg her husband's forgiveness. Parson Platt visits Fairfax and encourages him to take action against the Diggers for their immorality. Tom Haydon arrives at the commune with news that local shopkeepers will no longer trade with the Diggers.

Meanwhile, Fairfax commits his soldiers to support the eviction of the Diggers from St George's Hill. The Diggers flee and are attacked by the soldiers. Captain Gladman oversees the burning and total destruction of the settlement. Returning from the eviction, Platt is greeted with cheers and applause. Alone atop St George's Hill, Winstanley acknowledges that his struggle has ended.