this is the shortest sumary
Absalom and Achitophel is a landmark poetic political satire by John Dryden. The poem exists in two parts. The first part, of 1681, is undoubtedly by Dryden. The second part, of 1682, was written by another hand, most likely Nahum Tate, except for a few passages---including attacks on Thomas Shadwell and Elkanah Settle as Og and Doeg---that Dryden wrote himself.
The poem is an allegory that uses the story of the rebellion of Absalom against King David as the basis for discussion of the background to the Monmouth Rebellion (1685), the Popish Plot (1678) and the Exclusion Crisis.
The story of Absalom's revolt is told in the Second Book of Samuel in the Old Testament of the Bible (chapters 14 to 18). Absalom rebels against his father King David. The beautiful Absalom is distinguished by extraordinarily abundant hair, which is probably meant to symbolize his pride (2 Sam. 14.26). When David's renowned advisor, Ahitophel (Achitophel in the Vulgate) joins Absalom's rebellion, another advisor, Hushai, plots with David to pretend to defect and give Absalom advice that plays into David's hands. The result was that Absalom takes the advice of the double agent Hushai over the good advice of Ahitophel, who realizing that the rebellion is doomed to failure, goes home and hangs himself. Absalom is killed (against David's explicit commands) after getting caught by his hair in the thick branches of a great oak: "His head caught fast in the oak, and he was left hanging between heaven and earth, while the mule that was under him went on" (NRSV 2 Sam. 18:9). The death of his son, Absalom, causes David enormous personal grief. The title of Faulkner's novel Absalom, Absalom! is taken from David's mourning in 2 Sam. 18:33 or 19:4.
In 1681 in England, Charles II was in advanced years. He had had a number of mistresses and produced a number of illegitimate children. One of these was James Scott, the Duke of Monmouth, who was very popular, both for his personal charisma and his fervor for the Protestant cause. Charles had no legitimate heirs, and his brother, the future James II of England was suspected of being a Roman Catholic. When Charles's health suffered, there was a panic in the House of Commons over the potential for the nation being ruled by a Roman Catholic king. The Earl of Shaftesbury had sponsored and advocated the Exclusion Bill, but this bill was blocked by the House of Lords on two occasions. In the Spring of 1681, at the Oxford Parliament, Shaftesbury appealed to Charles II to legitimate Monmouth. Monmouth was caught preparing to rebel and seek the throne, and Shaftesbury was suspected of fostering this rebellion. The poem was written, possibly at Charles's behest, and published in early November of 1681. On November 24, 1681, Shaftesbury was seized and charged with high treason. A trial before a jury picked by Whig sheriffs acquitted him.
Later, after the death of his father and unwilling to see his uncle James II become King, the Duke of Monmouth executed his plans and went into full revolt. The Monmouth Rebellion was put down, and in 1685 the Duke was executed.