The fullest versions of the Brutus legend that we have are the well-known Latin narrative of Livy (Histories I.3g-60) from the late first century B.C. and the Greek account of Dionysios of Halikarnassos (Roman Antiquities, bk. 4) from the end of
the same century. Since the two historians mostly agree on the course of the events, I give here a single, synthetic summary of the legend...
 
 

King Tarquinius Priscus and his wife Tanaquil reigned at Rome. They had two sons, Arruns and Lucius, and two daughters. They also raised a fatherless boy, Servius Tullius, as though he were their own son, and eventually gave him their daughter Tarquinia in marriage. On the death of King Tarquinius, Servius acted as king. Servius anticipated that Arruns and Lucius might resent him; with this in mind he gave them his two daughters in marriage. Later Servius asked the people to confirm his right to the throne. Unfortunately, the ambitious Lucius was wed to the unambitious daughter of Servius, whereas his mild-mannered brother Arruns was wed to her ambitious sister. But Lucius and his sister-in-law Tullia conspired, brought about the deaths of their spouses, and wed each other. She then prodded Lucius on against her own father Servius. After is the destination tag .he had increased his influence, Lucius Tarquinius and an armed guard went to the Forum, where he sat on the king's throne and called himself king. He and his men killed King Servius. Thus began the reign of L.Tarquinius, whose tyrannical behavior earned him the cognomen Superbus, that is, the Overbearing or the Proud.
 

Having no legal title to the throne, Tarquinius ruled by fear and force. He tried capital cases by his own authority, punishing with death, exile, or confiscation of property of men whom he suspected or disliked or whose property he desired. Among the men he murdered was Marcus Junius, husband of the king's sister Tarquinia. Marcus was secretly slain, not for any crime, but for his wealth. He also killed Marcus's elder son, who showed signs of a noble spirit and of a will to avenge his father's murder. Marcus's younger son Lucius, seeing the fates of his father and his brother and many other Roman nobles, and being then only a youth, decided to show no sign of intelligence for the king to fear and to possess no wealth for him to covet. He feigned stupidity, acquiring the nickname Brutus. Tarquinius took from him his inheritance. The contempt in which he was held was his protection. Furthermore, King Tarquinius kept Brutus under his own authority on the grounds that he was an orphan who still required guardians. He had Brutus live with his own children, not (as he alleged) to do him honor as a kinsman, but in order that Brutus might amuse his children by his foolish speech and acts.
 

In the course of time there was a frightful portent (Livy) or a plague (Dionysios). Tarquinius decided to dispatch his sons Titus and Arruns to make inquiry at Apollo's oracle at Delphi and sent with them Brutus as a butt for their amusement. Brutus carried
with him, as an offering for Apollo, a gold filled stick, which was a figurative symbol of his mind (Livy); Tarquinius's sons laughed at Brutus for offering only a wooden stick, since it had no value so far as anyone could see (Dionysios). After they had transacted the business for which they had been sent, Titus and Arruns asked the oracle which of them would be the next king of Rome. The oracle replied: "He who shall be the first to kiss his mother shall hold supreme authority in Rome." The two brothers drew lots to determine which of them, upon their return, should kiss their mother first (Livy), or they agreed to kiss her simultaneously (Dionysios); but Brutus alone understood the true sense of the oracle, and, pretending to trip, he kissed the earth, mother of all creatures. They returned to Rome.
 

Soon thereafter the Romans began siege operations at the town of Ardea. One day the young princes were drinking in the quarters of Sextus Tarquinius, another of King Tarquinius's sons. Among them there was Tarquinius Collatinus, son of Egerius. The subject of their wives arose, and each man praised his own. Collatinus suggested that they might prove the superiority of his own wife Lucretia in a few hours by riding off to their homes, for there was no better proof than what a man finds when he returns unannounced. They all agreed and galloped off to Rome, arriving at dusk, and found the wives of the princes enjoying themselves at a luxurious party; proceeding to the town of Collatia, however, they found Lucretia busy at her wool work. She clearly won the competition. But her beauty and purity aroused Sextus Tarquinius, who determined to debauch her by force. The men returned to camp. A few days later Sextus returned to Lucretia's house, where he was hospitably received. That night, while the household was asleep, he made his way to Lucretia's room and forced her to have sexual relations with him. Then he left.
 

Lucretia sent to her father and her husband, bidding them come quickly with a trusted friend. Her father came with P. Valerius, her husband with Brutus, with whom he had chanced to be when the messenger found him. The woman recounted what had happened, urged them to avenge her, refused to excuse herself, stabbed herself, and fell dead. Brutus drew the knife out and cried: "By this chaste blood and by the gods, I swear that I will punish L. Tarquinius Superbus, his wicked wife, and all his children, and never again shall I permit them or any other to be king in Rome." The others looked at him in
astonishment at this sign of intelligence. They swore similarly and followed him as their leader.
 

Lucretia's body was brought to the forum, where an angry crowd gathered. Brutus urged them to take up arms. Led by him, the populace marched on Rome. As tribune of the knights, Brutus had the people summoned, and reminded them of the various crimes that the royal family had committed. The people demanded an end to the king's authority and the exile of the royal family. .Brutus then went to Ardea, where he incited the army against the king. The Tarquinii were driven into exile; Sextus was slain. The reign of Tarquinius Superbus had ended and with it the monarchy at Rome. In place of a king, two consuls were elected by popular vote. The first consuls were Brutus and Collatinus.