Although Old Arcadia has never been greatly popular, it has entertained a small set of readers for over 400 years with its sensational treatment of sex, politics, violence, soporifics, mobs, and cross-dressing. Narrated in sprawling Renaissance prose, the romance comprises five "books or acts," organized according to the five-part structure of classical dramaturgy: exposition, action, complication, reversal, catastrophe. This hybrid structure—part prose romance and part classical drama—allows Sidney to contain the diverseness of romance within the cohesiveness of the dramatic arc. The work is often called "tragicomic" for its combination of a "serious" high plot centering on the princes and Duke Basilius's household and a "comic" low plot that centers on the steward Dametas's family. The standard modern edition of Old Arcadia, on which this synopsis is based, is edited by Jean Robertson (Clarendon: Oxford, 1973).
In Book I, the Duke of Arcadia, Basilius, journeys to the oracle at Delphos and receives a bleak prediction: his daughters will be stolen by undesirable suitors, he will be cuckolded by his wife, and his throne will be usurped by a foreign state. Hoping to preempt this fate, Basilius entrusts the Arcadian government to his loyal subject, Philanax, and retires to a pastoral lodge with his wife, Gynecia, their attractive daughters, Pamela and Philoclea, his boorish servant, Dametas, and the latter's repulsive wife and daughter, Miso and Mopsa. In a nearby city, Pyrocles and Musidorus pass the night; they are cousins, princes, and best friends, and are famous throughout Greece for their heroic exploits. Pyrocles, upon seeing a picture of Philoclea at a gallery, is overwhelmed by a passionate desire to see her in person. To that end, Pyrocles disguises himself as Cleophila, an "[Amazonian lady] going about the world to practice feats of chivalry," and heads for Basilius's pastoral lodge, accompanied by the skeptical but loyal Musidorus. Deceived by Cleophila's feminine disguise, Basilius falls in love with her, and invites her to stay with the family. While Musidorus covertly observes this meeting, he is overwhelmed by a passionate love for the elder daughter, Pamela, and decides to disguise himself as a shepherd, Dorus, in order to gain access to her. When everyone congregates in an arbor to hear the shepherds sing, a lion and bear attack the party. Cleophila kills the lion, saving Philoclea; Dorus kills the bear, saving Pamela. Cleophila's manly puissance leads Gynecia to suspect her secret male sex, while Philoclea forms an intense "sisterly" affection for Cleophila.
In Book II, the action rises as the romantic relationships become increasingly complicated, and as Basilius's retirement foments political unrest. Cleophila struggles to woo Philoclea while simultaneously placating the jealous Basilius and Gynecia. Meanwhile, Dorus, who has ingratiated himself to Dametas and entered his household as a shepherd-servant, struggles to woo Pamela, who is always accompanied by Dametas's vain and ugly daughter, Mopsa. To avoid raising suspicions that he loves Pamela, Dorus addresses all of his significant looks, sighing, singing, poetry, etc. to Mopsa, who laps it up and fails to notice the heavy-handed hints that Pamela, not Mopsa, is the object of his passion. In an extremely complicated piece of hoodwinking, Dorus reveals his identity to Pamela, proposes elopement, and is elated by the princess's willingness to flee Arcadia with him. Meanwhile, Cleophila manages to reveal his identity to Philoclea, and they declare their mutual love. Their idyll is interrupted by a mob of drunken Arcadian rabble who are angry at Basilius for neglecting his sovereign obligations. Cleophila, Basilius, Dorus and friendly shepherds slaughter much of the mob before finally subduing it. Book II ends with the establishment of the unusual love "square" in which father, mother and daughter are all violently in love with the cross-dressed Pyrocles/Cleophila, and the love triangle comprising Mopsa, Pamela and Dorus. It also begins the political theme, expanded in the fourth and fifth books, concerning the implications of negligent government.
In Book III, Musidorus tells Pyrocles of his intentions to elope with Pamela. Pyrocles despairs of his own success with Philoclea because he is under the constant surveillance of the jealous and enamored Basilius and Gynecia. Dorus's elopement strategy begins by distracting Pamela's guardians: he tricks Dametas into wasting a day on a bogus treasure hunt; he dispatches Miso by telling her Dametas is cheating on her with a woman in an adjacent village, and he leaves Mopsa up in a tree waiting for a sign from Jove. The coast cleared, Dorus and Pamela head for the nearest seaport. While resting, Dorus is overcome by her beauty and is about to rape her when they are suddenly attacked by another mob. Meanwhile, Gynecia's passion has become desperate and she threatens to expose Cleophila's identity if he does not requite her love. To distract Gynecia, Cleophila pretends to requite her love, which aggravates Basilius and Philoclea. In an ill-fated bed-trick, Cleophila promises a nocturnal assignation to both Gynecia and Basilius in a nearby cave, intending to trick the husband and wife into sleeping with each other (hoping they won't notice that the other is not Cleophila) and to enjoy a night alone with Philoclea. Book III ends with a double "climax": the attempted rape of Pamela by Dorus, and the consummated union of Pyrocles and Philoclea.
In Book IV, Dorus and Cleophila suffer a major reversal of fortune. Dametas, Miso and Mopsa return to the lodge to find Pamela missing, and Dametas, fearing punishment for neglecting his royal ward, begins a frantic search for Pamela. Supposing her to be with her sister, Philoclea, Dametas barges into Philoclea's bed chamber and finds, of course, not Pamela, but "Cleophila," who is naked and in bed with Philoclea, and who is evidently not a woman. Dametas bolts the lovers inside and sounds the alarum. In the cave, Gynecia and Basilius, each thinking the other is Cleophila, have sex, but recognize each other in the morning. Basilius accidentally drinks a potion that Gynecia had intended for Cleophila, and dies. Gynecia becomes hysterical and self-incriminating, and offers herself up to justice as the murderer of her husband and the sovereign. Philanax, Basilius's loyal friend, arrives to investigate the duke's death and Pamela's flight, and becomes a zealous advocate for executing everyone associated with the death of Basilius. Meanwhile, Musidorus and Pamela fall captive to the attacking mob, but not before Musidorus kills and gruesomely maims several of them. Hoping for a reward for finding the fugitives, the mob heads for Basilius, but is intercepted and slaughtered by Philanax and his men, who take the captive Dorus and Pamela, who are now primary suspects in the duke's murder. Thus, Dorus (now "Palladius") joins Cleophila (now "Timopyrus") in prison. Pamela demands to be recognized as the new sovereign, but Philanax demands an interim period of investigation and burial before the succession is established. Meanwhile, the body politic erupts into a confused and "dangerous tumult" about political succession.
Book V brings the action to its catastrophe. Philanax struggles to maintain order in Arcadia, which is dangerously divided: some factions support various political climbers, others clamor for democratic government, and some call for the election of the two princes, whose good looks and military prowess had made them very popular. Philanax needs a leader capable of commanding the allegiance of the Arcadians and of bringing justice to Basilius's murderers. Luckily, the sovereign most renowned for his wise and just government, Euarchus of Thessalia, has traveled to Arcadia to visit his good friend Basilius. Euarchus is also the father of Pyrocles and uncle of Musidorus, but has no idea what they have been up to. Philanax persuades the reluctant Eurarchus to aid Arcadia by assuming authority for the present and becoming the state's "protector." The book concludes with a lengthy trial scene. Gynecia, "Palladius" and "Timopyrus" are brought forth to stand before Euarchus, who presides as judge, and Philanax, who argues on behalf of the apparently murdered Basilius. Gynecia's trial goes quickly because she, overcome by grief, wants to die as quickly as possible, and gives a false confession of intentionally poisoning her husband and sovereign. Euarchus sentences her to death by being entombed alive with Basilius. "Timopyrus" is tried next, and Philanax delivers a vituperative oratory condemning him for cross-dressing, for raping Philoclea, and for conspiring with Gynecia to murder Basilius; "Timopyrus" is acquitted of the murder charges, but is sentenced to death for raping Philoclea. "Palladius" is likewise condemned to death for attempted theft of the royal daughter, Pamela. As the convicts are escorted to their executions, a friendly compatriot of Musidorus suddenly arrives with important information. He has heard about the trial, guessed the princes' true identities, and feels Euarchus should know that he has condemned his own son and nephew to death (for various reasons, the identities of Euarchus and the princes has been hitherto obscured). At this moment of recognition, or anagnorisis, Euarchus is devastated, but decides that justice trumps kinship, and with a heavy heart confirms their death sentence. Suddenly, groans are heard from Basilius's corpse and, to the surprise and delight of all, Basilius emerges from a deep coma. All are forgiven, the princes marry the princesses, and the book thus ends with a comic reversal, or peripeteia, from justice and death to reconciliation and marriage.