Geoffrey of Monmouth places Brennius in the legendary succession after Dunvalo Molmutius, whose name also appears as Dunwallo or Dunvallo. The king leaves two sons, Belinus the elder and Brennius the younger. Their followers initially negotiate a division: Belinus receives the crown and the greater part of Britain, while Brennius holds the northern lands under his brother. The younger brother does not simply reject the crown in favour of peaceful independence. His advisers encourage him to challenge his subordinate position, and a marriage alliance with Norway helps turn the dispute into war.

Belinus takes control of the northern territory while Brennius is abroad. Storms, rival rulers and the capture of Brennius's bride complicate his return before the brothers fight at Calaterium. Belinus wins, and Brennius escapes across the sea. In Gaul he finds protection with Segin, the duke of the Allobroges, whose favour eventually brings him marriage to the duke's daughter and succession to his lands. This continental connection transforms the defeated exile into a ruler capable of raising another army. Geoffrey's geography and institutions belong to a medieval reconstruction of antiquity, not a reliable political map of pre-Roman Europe.

Brennius returns to Britain intending to recover what he has lost. Before another battle begins, his mother Conwenna approaches him and appeals to their family bond. Geoffrey makes her intervention the turning point: Brennius listens, embraces Belinus and abandons the immediate struggle against him. The brothers become partners in a new campaign across the Channel. Their reconciliation ends one war but makes a much larger expedition possible, carrying the narrative from a family dispute into Geoffrey's imagined history of British conquest abroad.

The brothers advance through Gaul and towards Rome. Geoffrey names the Roman leaders Gabius and Porsenna and describes a settlement involving tribute and hostages. When the Romans later attack, Belinus ambushes them and joins Brennius in renewed warfare. The brothers besiege Rome, execute hostages before its walls and eventually take the city. Their stories then separate. Belinus returns to Britain and is remembered as a builder and lawgiver, while Brennius remains in Italy. Geoffrey characterises his subsequent rule as tyrannical, making the brothers' different endings part of the chronicler's moral contrast rather than an independently verified judgement about ancient rulers.

Brennius must be distinguished from the Gallic Brennus in Roman accounts of the sack of Rome. Livy's Book V tells the famous story of Brennus adding his sword to the ransom scales and declaring Vae victis, usually translated as woe to the conquered. That is an episode in Roman historical tradition, not evidence that Geoffrey's British prince personally uttered the words. Similarities between the names and conquest stories invite comparison, but they do not justify merging Brennius with every Brennus or with the Welsh Bran. His place in Mythopouch is as the legendary brother of Belinus, a figure in the British chronicle tradition whose story joins rivalry, exile, reconciliation and conquest.