King John enters his court with his mother, Queen Eleanor, the lords Pembroke, Essex, and Salisbury, and the French messenger Chatillon. John asks what messenger what he has to report. Chatillon says that the French King Philip speaks on behalf of John's elder brother's son Arthur, and he declares Arthur's legal claim to the throne of England and its Irish and French territories. Philip asks John to abdicate in favor of Arthur, and Chatillon insinuates that if John refuses France will declare war on England. John urges Chatillon to hasten back to France under threat of violence from England; he adamantly refuses to give up the throne.
Eleanor says that Arthur's mother, Constance, must be behind this development and that she expected that Constance would stir up such trouble. John says his strong possession of the throne will aid him in future conflicts, and Eleanor agrees, though she hints that his right to the throne may be questionable. A sheriff enters leading two men who want to present a strange conflict. Falconbridge and Philip the Bastard enter. The Bastard explains that he is the eldest son of the deceased Robert Falconbridge. Falconbridge explains that he is the second son of that same Robert Falconbridge and claims to be his legitimate heir. The Bastard suggests that he and his brother had different fathers, but Eleanor scolds him for speaking so ill of his mother. It's his brother's fault, he explains, because his brother lays claim to the inheritance, declaring him to be a bastard. John asks Falconbridge why he makes such claims.
Falconbridge explains that his father was away in Germany for a long time, while the former king, Richard the Lionhearted, stayed at his father's estate. His father was convinced that the Bastard was not his son and on his deathbed willed his lands to his younger son. John points out that the elder Falconbridge raised the Bastard as his son, thereby accepting him as his eldest son; a wife's sons become the husband's heirs, legally. Therefore whether or not he is Richard the Lionhearted's son, he is the elder Falconbridge's heir. Falconbridge asks if his father's will is not enough to prove his father's desire to keep the lands from the Bastard.
Eleanor interrupts and asks the Bastard if he would rather claim his name as the bastard son of Richard the Lionhearted, without land, or be a Falconbridge and enjoy his property. The Bastard notes that he doesn't look anything like the elder Falconbridge, so no one would believe he was the Falconbridge heir. Eleanor, impressed with the Bastard, asks him if he would rather give his land to his brother and follow her in an attack on France. The Bastard quickly agrees. John knights the Bastard, renaming him Sir Richard Plantagenet.
Everyone departs except for the Bastard, who contemplates his change of fortune. Now a knight, he expects his ambition will flourish. Flattery will aid his ascent, he believes, but he must beware of deceitful flattery. Then Lady Falconbridge and her attendant enter; she seeks Robert Falconbridge, the Bastard's brother, to scold him for dragging her reputation through the mud. The Bastard announces that he is not her husband's son and demands to know who his real father is. She asks him if he has conspired with his brother to scandalize her. He says he has given up the Falconbridge title and lands. She says that his father was Richard the Lionhearted, who seduced her when her husband was away. The Bastard assures her that he doesn't think she sinned, for he could not wish to have a better father. He promises her that he will fight with anyone who says she sinned in giving birth to him.
Hereditary legitimacy–the validity of the passage of land, title, or position to children from their deceased parents, according an elaborate code of social rules–is a main concern in King John and is brought up in this first act in the figures of both John and the Bastard. John's lineage is undoubted; he is the third son of Henry II, who was the father of Richard the Lionhearted, the previous king. When Richard died childless, the throne legally should have passed to the eldest brother of the deceased king or the eldest brother's children. Arthur is the son of Richard's eldest remaining brother and legally should be king. John, on the other hand, stakes his claim on the throne through being the third son of Henry II. His mother–Henry II's wife, Eleanor–supports him, and his claim to the throne is based on his personal strength compared to Arthur's relative personal weakness. Yet Arthur has found a champion in Philip, the King of France.
John's royal lineage is not in question, but what he can legally claim to possess based on that lineage is in doubt. The Bastard, on the other hand, is legally entitled to inherit the lands of his foster father. His father's deathbed will cannot move the law, which says that the offspring of a wife's affair is the legitimate son of her husband. Surprisingly, being a bastard child is not a barrier to inheritance; the Bastard can become a landed squire in place of his brother, an actual blood child of the Falconbridge line. Apparently being in the right position is vital to gaining possessions legally, even if one's lineage is in question. The Bastard is in the right spot to overturn his scandalous birth, but Arthur, whose lineage is in order, is not in the right place to claim the throne of England.
However, the Bastard turns down the inheritance, choosing to become a landless knight known as the bastard son of Richard the Lionhearted rather than a landholding gentry with the name of Falconbridge. His ambitions are larger than those of a mere landholder, and becoming a knight with the royal name Plantagenet pleases him more.
Unlike many of the characters in King John, the Bastard is not an actual historical figure, and in many ways he is less a coherent character than a set of theatrical functions. Shakespeare based him in part upon the vice figure, a mischievous allegorical character common in earlier English morality plays. The vice figure combined a commitment to evil with an intimacy with the audience and an alluring sense of fun. In asides and soliloquies, he denounces the failings of the royals while he gleefully announces his subscription to their self-interested schemes. However, later in the play the Bastard becomes one of the more responsible figures, proving himself an ethical center in a play largely without a rhetoric of positive values. The Bastard becomes the most vital and most interesting character in the play.
Lady Falconbridge arrives with the intention of defending her honor against the claims her son makes against her, first bringing up the play's concern with the uncertainty of biologically legitimate patriarchal succession. Hereditary descent from father to son requires wives to be sexually faithful to their husbands–but no father can ever be completely sure of their sons' paternity. The role of women, therefore, is necessary to hereditary lineage, but it is also a potential threat. This anxiety is later reflected in vicious exchanges between Constance and Eleanor when they accuse each other of infidelity. These mothers offer potential damage to their sons even after their lineages are assured, as we see through their micromanagement of the careers of John and Arthur, and the fact that both sons seem to weaken considerably after the deaths of their mothers.