The story begins with a wound. The Fourth Crusade had shattered the Byzantine Empire in 1204, and the Aegean, once Byzantium's naval heartland, fragmented into a patchwork of Latin lordships under the watchful protection of Venice. When Michael VIII Palaiologos recaptured Constantinople in 1261 and restored the empire, he knew his throne floated on water. He courted Genoa against Venice, then made peace with Venice, and above all he built ships. By the 1270s a revived Byzantine navy was probing the Latin-held islands, slowly clawing back fortresses around Euboea, known then as Negroponte, with the help of a Latin renegade named Licario who had switched to the imperial side.

In the early 1270s Michael VIII launched a major land campaign against John I Doukas, the ruler of Thessaly, led by his own brother, the despotes John Palaiologos. To keep the Latin princes from sending help, he dispatched a fleet of seventy-three ships under Alexios Philanthropenos to raid their coasts and bottle them up. The plan unraveled. The Byzantine army was beaten at the Battle of Neopatras, aided by troops from the Duchy of Athens. When word of that defeat spread, the Latin barons of Euboea and Crete took heart. The imperial fleet lay at anchor in the harbor of Demetrias, distracted and vulnerable, and they resolved to strike it before it could recover.

The exact numbers are lost to contradicting chronicles. The Byzantine historian Nikephoros Gregoras counted over fifty imperial vessels; the Italian Marino Sanudo said eighty. The combined Latin fleet of Lombard and Venetian ships was smaller, perhaps thirty to sixty, outnumbered by roughly a third. They made up the difference with surprise and engineering, building tall wooden fighting towers on their decks that let their crews shoot and fight down onto the lower Byzantine ships. The opening attack was ferocious. Many Byzantine sailors and soldiers were killed or drowned in the crowded harbor, pitched into the water as the towers rained down men and missiles, and victory seemed to be slipping into Latin hands.

Then came the rescue. Retreating from the disaster at Neopatras, John Palaiologos had learned that his fleet was in danger. He did not wait for orders or reinforcements. He scraped together every man he could find, took to boats, and rowed through the dark, covering forty miles in one night to reach Demetrias just as the imperial fleet began to waver. His arrival tipped the scale. Fresh crews and renewed nerve turned a near-rout into a crushing Byzantine victory. For the men who drowned that day, Latin and Byzantine alike, the gulf was the last thing they saw, but the battle is remembered for the one commander who refused to let the night, or forty miles of open water, decide the outcome for him.