Hrútr and Hallgerðr
The first episode covers the period from the betrothal of Hrútr Herjólfsson and Unnr to the ugly legacy of their divorce. We are shown Hrútr's exploits in Norway, where he gains honour at court and in battle, but he ruins his subsequent marriage by becoming the lover of the aging queen mother Gunnhildr. When he denies having a woman in Iceland, she curses him so that he is unable to consummate his marriage. After Unnr divorces him, he retains the dowry by challenging Unnr's father, Mörðr, to combat. Mörðr refuses, as he knows Hrútr's reputation and that he will lose the fight. Because of this, Hrútr keeps the dowry. While this conforms to Icelandic law, it offends justice.
The first chapter gives one of Hrútr's insights when he comments of his beautiful niece, "I do not know how thieves' eyes came into the family". The saga next follows this niece, Hallgerðr, through her first two marriages. Both husbands die by the axe of Hallgerðr's doting, brutish foster-father, Þjóstólfr. Hallgerðr provokes the first death but not the second, although it follows from a disagreement between her and her husband. It is Hrútr who, despite the family ties, avenges the death by killing Þjóstólfr.
Gunnarr and Njáll
Gunnarr Hámundarson and Njáll Þorgeirsson are now introduced. Gunnarr is a man of outstanding physical prowess, and Njáll has outstanding sagacity; they are close friends. When Gunnarr is obliged to revive Unnr's dowry-claim against Hrútr, Njáll gives him the means to do so. By skillful play-acting, Gunnarr begins the legal process in Hrútr's own house. He follows Hrútr's doubtful example when it comes to court, and Hrútr, who has previously won by threat of violence, loses to a threat of violence. Despite his humiliation, he sees future links with Gunnarr.
This comes about when Gunnarr returns with honours from a trip to Scandinavia. He goes to the Althing – the annual assembly – in splendour, and meets Hallgerðr. They are impressed with one another and are soon betrothed, despite Hrútr's warnings about Hallgerðr's character, and Njáll's misgivings.
Hrútr and Njáll are proven right when Hallgerðr clashes with Njál's wife, Bergþóra. Hallgerðr charms a number of dubious characters into killing members of Njáll's household and the spirited Bergþóra arranges vengeance. After each killing, their husbands make financial settlements according to the status of the victims. The fifth victim is Þórðr, foster-father of Njáll's sons. Þráinn Sigfússon, Gunnarr's uncle and Hallgerðr's son-in-law, accompanies the killers. When the feud ends and settlements are made, Þráinn’s presence at that killing later causes conflict.
Hallgerðr now uses one of her slaves, Melkólfr, to burgle the home of a churlish man named Otkell. Gunnarr immediately seeks to make amends, but his handsome offers are not accepted. A lawsuit is started against him which, with Njáll's help, he wins, gaining great honour. However, while remonstrating with Hallgerðr about the burglary, Gunnarr slaps her.
This is followed by Otkell accidentally wounding Gunnarr. Insult follows injury and Gunnarr reluctantly goes to avenge himself. With belated help from his brother Kolskeggr, he kills Otkell and his companions.
Under Njáll's influence a new settlement is arranged, and Gunnarr's reputation grows. Njáll warns him that this will be the start of his career of killings.
Next, Gunnarr accepts a challenge to a horse-fight from a man called Starkaðr. In the course of the fight, his opponents cheat, and Gunnarr finds himself in a fresh squabble. Njáll tries to mediate but Þorgeir Starkaðsson refuses to accept it. On a journey with his two brothers, Gunnarr is ambushed by Starkaðr and his allies. In the battle, fourteen attackers and Gunnarr's brother Hjörtr are killed.
Worming through all this is Unnr's son, Mörðr Valgarðsson. Mörðr envies and hates Gunnarr, and uses other men to attain his aims. He has learned that Njáll prophesied that Gunnarr will die if he kills twice in the same family. He instigates an attack on Gunnarr by persons dissatisfied by the settlement. Again, Gunnarr wins the fight, but he kills a second man in the same family. The settlement that follows requires that Gunnarr and Kolskeggr leave Iceland for three years.
Arrangements are made for exile. But as Gunnarr leaves home, he looks homeward and, touched by the beauty of his homeland, resolves not to leave Iceland, thus becoming an outlaw. He goes about as though nothing has changed but his enemies, Mörðr among them, seek revenge. He defends himself in his home until his bowstring is cut. Hallgerðr refuses to give him strands of her hair to restring his bow; this is in revenge for the slap he once gave her. Some readers choose to interpret this episode as her forgiveness since human hair is unusable as bowstring; i.e. he asks for something he knows is useless and she answers by denying as revenge, fully knowing too. Gunnarr's enemies resist Mörðrs proposal to burn him in the house as shameful, but eventually they take the roof off to get to Gunnarr. Njáll's son Skarp-Heðinn assists Högni Gunnarsson in some acts of vengeance before a settlement is achieved.
Kári and the sons of Njáll
Scandinavian rulers honor two Icelandic expeditions: those of Þráinn Sigfússon and of Njáll's two younger sons. Both return with enhanced honor, but also with companions. Þráinn brings back the malevolent Hrappr; the sons of Njáll and the noble Kári Sölmundarson, who marries their sister. But Njáll's sons also bring back a grievance, blaming Þráinn for the way in which the de facto ruler of Norway, Jarl Hákon, has treated them while looking for Hrappr, who had been hidden by Þráinn. While Njáll says they have been foolish in raising the matter, he advises them to publicise it so that it will be seen as a matter of honor. Þrain refuses a settlement, and his retainers, including Hallgerðr, on her last appearance, insult them.
The most dramatic of the saga's battles follows. Njáll's sons, with Kári, prepare to ambush Þráinn and his followers. There is a bridge of ice over the river between them. Skarp-Heðinn overtakes his brothers, leaps the river, and slides on the ice past Þráinn, beheading him in passing. Between them the attackers kill four men, including Hrappr.
Þráinn's brother, Ketill, has married Njáll's daughter, and between them they bring about a settlement. Wishing to stop further contention, Njáll adopts Þráinn's son Höskuldr as his foster-son. Höskuldr grows up in Njáll's household, and is loved and favoured by him. When he is fully grown, Njáll attempts to find a suitable wife for him, Hildigunnr. However, she refuses, saying that she will only marry Höskuldr if he becomes a chieftain. Njáll manages to get Höskuldr a chieftaincy by instituting the Fifth Court at the Althing, and Höskuldr and Hildigunnr are married.
At this point the saga recounts the conversion of Iceland to Christianity in AD 999.
Höskuldr and Flosi, the burning
Mörðr Valgarðsson now finds Höskuldr to be such a successful chief that his own chieftaincy is declining. He sets the sons of Njáll against Höskuldr; the tragedy of the saga is that they are so susceptible to his promptings that they, with Mörðr and Kári, murder him as he sows in his field. As one character says, "Höskuldr was killed for less than no reason; all men mourn his death; but none more than Njal, his foster-father".
Flosi, the uncle of Höskuldr's wife, takes revenge against the killers, and seeks help from powerful chieftains. He is pressured (against his better judgement) by Hildigunnr to accept only blood vengeance. Njáll's sons find themselves at the Althing having to plead for help. Skarp-Heðinn has become grimly fatalistic, and insults many who might help them.
After some legal sparring, arbitrators are chosen, including Snorri goði, who proposes a weregild of three times the normal compensation for Höskuldr. This is so much that it can only be paid if the arbitrators, and many at the Althing, contribute. The great collection is gathered, and Njáll adds a gift of a fancy cloak. Flosi claims to be insulted by the offer of a unisex garment (an insult from Skarp-Heðinn also adds fuel to the fire) and the settlement breaks down.
Everyone leaves the Althing and prepares, amid portents and prophecies, for the showdown. A hundred men descend on Njáll's home, Bergthorsknoll (Bergþórshváll), to find it defended by about thirty. Any victory for Flosi will be at some cost. But Njáll suggests that his sons defend from within the house, and they, while realizing that this is futile, agree. Flosi and his men set fire to the building.
Both the innocent and the guilty are surrounded. Flosi allows the women to leave but beheads Helgi Njálsson, who attempts to escape disguised as a woman. Although Flosi invites Njáll and Bergþóra to leave, they refuse, preferring to die with their sons and their grandson Þórðr (the son of Kári). Eventually eleven people die, not including Kári who escapes under cover of the smoke by running along the beam of the house. Flosi knows that Kári will exact vengeance for the burning.
At the Althing, both sides gather. Flosi bribes Eyjólfr Bölverksson, one of the finest lawyers in Iceland, into taking over the case, while his opponents blackmail Mörðr Valgarðsson into prosecuting, advised by Þórhallr, Njáll's foster-son, who was trained in the law by Njáll, but is kept away from the proceedings by an infected leg. There is a legal joust between the parties. Eventually, when his legal action seems to be failing, Þórhallr lances his boil with his spear and begins fighting. Flosi's men are driven back until Snorri separates the parties. In the confusion, several are killed including Ljótr, Flosi's brother-in-law.
Ljótr's father, Hallr of Síða, takes advantage of the truce to appeal for peace, and seeks no compensation for his son. Moved by this, all but Kári and Njáll's nephew Þorgeir reach a settlement, while everyone contributes to Ljótr's weregild, which in the end amounts to a quadruple compensation. The burners are exiled.
Before the sons of Sigfús reach home, Kári attacks them, and most of the rest of the saga describes his vengeance for the burning. He is supported by Þorgeir and an attractive anti-hero named Björn. He pursues them to Orkney and Wales. The most dramatic moment is when he breaks into the earl's hall in Orkney and kills a man who is giving a slanderous account of those killed at the burning.
After a pilgrimage to Rome, Flosi returns to Iceland. Kári follows, and is shipwrecked near Flosi's home. Testing Flosi's nobility he goes to him for help, and they arrange a final peace. Kári marries Höskuldr's widow. Finally, there is a full reconciliation.