Phineus belongs to the complicated family of Belus, a king connected with Egypt and Libya in Greek genealogy. Pseudo-Apollodorus says that Belus and Anchinoe had Danaus and Aegyptus and adds that Euripidean tradition also made Cepheus and Phineus their sons. That makes Phineus the brother of Cepheus, the king whose daughter Andromeda becomes the centre of the Perseus story. Other ancient genealogies arrange the family differently, so the Belus relationship belongs to one important branch rather than a single universal family tree.

Before Perseus arrives, Phineus is betrothed to Andromeda. The crisis begins when Cassiopeia, Andromeda's mother, boasts that she or her daughter surpasses the Nereids in beauty. Poseidon answers the insult with a flood and a sea monster. An oracle declares that the disaster will end only if Andromeda is exposed to the creature. Cepheus has his daughter chained to a rock beside the sea.

Phineus does not save her. Perseus, returning from the killing of Medusa, sees Andromeda and offers to rescue her in exchange for marriage. He defeats the sea monster, frees the princess and receives the promise of her hand. In the logic of the story, action replaces prior claim. The man who risks his life for Andromeda wins the marriage that Phineus expected by arrangement.

The conflict erupts at the wedding feast. Ovid gives the fullest surviving version. Phineus enters the palace carrying a spear and accompanied by a large armed following, announcing that he has come to avenge the theft of his bride. Cepheus rebukes his brother and reminds him that Perseus did not take Andromeda from him. The sea monster and the oracle had already taken her beyond Phineus's protection, and Perseus alone acted when the kingdom needed a rescuer.

Words become violence. Phineus throws his spear, and the banquet hall turns into a battlefield. Men fight with swords, spears, furniture and pieces of the building itself. When Perseus is surrounded, he warns his allies to look away and raises the severed head of Medusa, the Gorgon whose gaze turns living beings to stone. One attacker after another freezes in the pose of battle until the hall is crowded with statues.

Phineus escapes the first sight of the Gorgon because he turns away. Finding his followers transformed, he loses the confidence with which he entered the feast. He admits his fault, yields his claim to Andromeda and begs only for life. Perseus refuses the plea. Instead of killing him with a weapon, he moves Medusa's head into Phineus's line of sight. The prince hardens into stone with the fear and guilt of his final moment preserved on his face.

Pseudo-Apollodorus compresses the entire episode into a short notice in which Phineus conspires against Perseus and is turned to stone with his companions. Hyginus preserves a variant suitor named Agenor. The details therefore move between sources, but the central structure survives: an earlier claimant refuses to act during Andromeda's danger, challenges the man who rescued her and is defeated by the supernatural weapon that Perseus brought home from his earlier quest.