While the queen is seated on her throne, with the English lion at her feet, Duessa (Mary Queen of Scots) is brought before her and is proved guilty of countless crimes; but, although she evidently deserves death, Mercilla, too merciful to condemn her, sets her free.
It is while sojourning at Mercilla's elegant court that Artegall and Arthur see two youths appear to inform the queen that their mother Belge, or Belgium, a widow with seventeen sons, has been deprived of twelve of her offspring by a three-headed monster, Gereones (the personification of Philip the Second of Spain, the ruler of three realms).

This monster invariably delivers his captives into the hands of the Inquisition, by which they are sorely persecuted.

Hearing this report, Arthur steps forward, offering to defend the widow and her children.

Mercilla granting his request without demur, Arthur hurries away, only to find that Beige has been driven out of her last stronghold by a faithless steward (Alba).

But, thanks to Arthur's efforts, this steward is summoned forth, defeated in battle, and the lady reinstated in her domain.
Gereones now dauntlessly attacks Arthur, whom the giant Beige secretly instructs to overthrow ah idol in the neighboring church, as that will enable him to triumph without difficulty.