Unlike Shakespeare's earlier history plays, King John does not portray a providential movement of history, where everything happens for a reason on a predestined path to a moral conclusion. While the play focuses on some of the historical events of King John's reign, it also delivers less narrative drive than plays such as Henry V. Events in the plot disrupt the connection between intention and outcome throughout the play–the characters are thwarted by historical accident and adversity, making King John more a pragmatic representation of political events than a story shaped according to aesthetic ends.

The main conflict turns on John's efforts to retain the crown against claims that he is not the rightful heir to the throne. The opening scene of the play shows a struggle over inheritance between the Bastard and his younger brother, which leads to a surprising conclusion that being a bastard is not a barrier to inheritance. However, the Bastard then renounces his inheritance, choosing to be a landless knight.

Agreements come and go throughout the play. The English and French battle to stalemate while seeking the allegiance of Angers, then band together to destroy the town, before ending their quarrel and sparing the town by negotiating a marriage between heirs of France and England. But this resolution is transitory; the messenger from the pope, Pandolf, excommunicates John and insists King Philip of France, just joined to John's family in marriage, go to war against John.

Philip supports Arthur as the legal heir to the throne, so John thinks he can secure his hold on the throne by ordering Arthur killed. However, this assassination turns his lords against him and brings on an invasion by the French. Yet Arthur's executioner has actually spared Arthur, so John tries to reverse the situation. But Arthur dies in an accident that is interpreted as murder, and John's lords join the French army. John tries to undo the coming battle by belatedly submitting to the pope, but this has no effect. John's lords return to him when they hear the French plan to kill them.

Climactic battles take place offstage or not at all, derailed by last-minute treaties or a succession of armies lost at sea and drowned in tides. John dies away from the battlefield, poisoned by a monk angered by his robbery of the monasteries, making an undramatic ending based on circumstances barely portrayed within the scope of the play. The king's son appears conveniently at John's deathbed, just in time to announce the arrival of a peace accord from France. The ending seems orthodox enough–a dead king is succeeded by his son and heir–but it feels quite shaky, given that prospects for peace seemed so tremulous before John's death and that John was never proved to be the rightful heir anyway.

The play dramatizes several topics that would have interested Shakespeare's contemporary audience: a struggle with the papacy, the danger of invasion, and the debate about legitimate rule. These same topics were hotly debated during Queen Elizabeth's reign. Yet King John differs from Shakespeare's other histories. It portrays the thirteenth century rather than the fourteenth or fifteenth, and unlike other historical plays that were part of a series, this play stands alone. Other historical plays focused attention on the balance of power between the nobility and the king, and gave account of popular unrest; this play, by contrast, completely marginalizes the populace and does not attribute much strength to the nobles.

The real focus of the play thus becomes the question of legitimacy and fitness to rule, which turns on the relationship between John and Arthur. Arthur was the son of the previous king's eldest brother, making him the rightful heir, but John was chosen to rule by the previous sovereign. Yet in the case of the Bastard, John rules that a will cannot take precedence over the law; in that case, the father's will that his younger son receive the inheritance was overturned by the law, which stated inheritance must go to the eldest son, bastard or not. By ruling such, John unwittingly proves his own illegitimate hold on the throne, because it is based on will and not the legal right of succession.

Shakespeare proves that John is not the legitimate ruler, yet the question is complicated in the clear difference that develops between the idea of "legitimate" and "fit." Arthur is the legitimate ruler, but his portrayal as a weak child under his mother's thumb shows him to be unfit; that is, he would be a weak and ineffectual king. Because John is a stronger man, his claim on the throne begins to seem much more attractive.

This situation all gives rise to a kind of defense of illegitimacy. Toward that end, the Bastard develops as the most compelling character in the play. He enters less as a character than as a set of theatrical functions, embodying the mischievous vice figure of earlier English morality plays. He speaks to the audience and makes observations about events. Yet by the second half of the play, he becomes unswervingly loyal to the king, denouncing deals made between John and Philip, and between John and Pandolf, and criticizing the royal desire for "commodity" and self-interest. The Bastard seems to believe that Arthur's death was an accident and returns to John to defend the crown and kingdom. At this point he becomes both the rhetorical and ethical center of the play.

By supporting John, the heroic and honorable Bastard makes it look like John must be the right choice for king. But ordering the death of Arthur has tipped the balance between rightful rule and hereditary legitimacy in John's reign, and his unnecessary cruelty makes him seem unfit to rule. As the central argument is weakened, so is the hero of the play; the Bastard loses his armies in yet another watery grave, and he still wants to fight an irrelevant war with France after the others have already negotiated peace. He is not completely pushed aside–he makes the final speech of the play–but while he cheers on the unconquerable force of his nation, his resolution has less to do with victory than with the well-timed collapse of both opposing forces. And while he delights in England's power, he also notes that internal conflicts could yet doom the nation.