Mallecyne St. Merline and Guinevere Tintagel are both police officers in Isis City, in an alternate earth (San Francisco). Mallecyne is one of the most gifted of her generation in the sorcerous arts, a direct descendent of the great St. Merlin himself. A favored daughter of the goddess Diana, who has actually appeared to her protegee (though rarely), Mallecyne has made great progress in the police department, while still in her late twenties.
When she is assigned a new, special partner from another division for an assignment, she wonders just who this Hero Anglesey is. Gifted sorcery students have been disappearing from Morgana La Faye high school through some kind of a portal into another world, and Mallecyne and Hero are expected to find out why and where they have gone. When Mallecyne discovers that Hero is a Null, possessing no magical powers, she wonders how he could have risen so high in the department. But Hero is one of a family of Nulls with special powers, a family called Wizardsbane -- Hero is immune to the effects of magic, and cannot be harmed or controlled by any kind of spell or power. This makes him more powerful even than the legendary Tynedale, most renowned of the wizards who was exiled to a small island in the South Pacific for his crimes against the World Parliament.
Another of the students gets through the portal before Mallecyne and Hero can stop him -- the portal possesses a strong attraction for anyone with magical powers, and Mallecyne feels the urge to cross through it herself. Hero prevents her, and discovers that his nephew, Thor Wizardsbane, is a student at the high school.
Before they can pursue the case, Guinevere and her partner Mayne are kidnapped by 'Sand Dwellers' -- strange, nomadic people who live in the deep desert south of the land of Carthage. They possess no magic and follow a religion that believes in only one god, a male one at that. They are shunned by the rest of society and live poor lives, with little medical care and a high mortality rate. Mallecyne and her boss, Minerva Cerridwen, cannot imagine how these people managed to overcome Guinevere, a class five sorcerer, and Mayne, admittedly less of a magic talent but a formidable man nonetheless.
Mallecyne and Hero go to Carthage to consult with the police department there. It is believed that Guin and Mayne are on a ship that will arrive in the famed Carthaginian harbor in a week's time. They discover another of the strange portals in Carthage, this time in the Temple of Demeter. And this portal has the added feature of being something like a movie screen -- they can see into the other dimension, at least one room of it. There they view men who are strangely-dressed, and although they speak the universal language, it is oddly inflected.
When they manage to capture one of the people from the other dimension, Mallecyne and Hero discover that their young students are being lured through the portals to their deaths -- hideous deaths by torture at the hands of the strangely-dressed men.
Mallecyne and Hero have been attracted to one another from the beginning of being assigned together, and become lovers, despite the disapproval of their boss.
They are able to rescue Guinevere and Mayne, but it is Hero who proves to be the most valuable member of the team. They combine their forces with the Chief of Police of Carthage, and hide out at the enormous docks of the city in order to ambush the black sailing ship when it arrives in port. The slave-traders have some strange statuettes made of blue glass which render anyone with magical powers weak and drained, and most of the police are effected by them except Hero, who shoots their leader in the leg. He is revealed to be Alain Tynedale, Master Sorcerer, Mallecyne's professor and lover when she was in college. Although he has long been her nemesis, she is distressed by his being wounded, and tries to heal him. He disappears and takes her with him.
Hero determines that Tynedale has something to do with what is happening through the portals, and he and Mayne cross over to the other side in order to rescue Mallecyne and attempt to discover how to close the portals. They are able to overcome or kill the men on the other side of the Carthage portal, and discover, in the basement of the building on the other side, a lot of people who were captured by the strange men. Strangest of the 'others', as Hero calls them, is a man named Reverend Binny who totes around a statue of a crucified man. This seems bizarre in the extreme to both Hero and Mayne, who at first assume he is part of some strange cult of human sacrifice, like the worshippers of Baal Hammon (now outlawed in their own world). They soon discover, however, that on this alternate Earth Christianity and the other patriarchal religions didn't die out but have rather taken over, with a vengeance. They have landed in a small town called Porterville, near to Boston, sometime in the middle of the 1700's. The witch scare is in full-force, and Tynedale has convinced the people of the village that all those who come through the portal are witches.
Hero and Mayne rescue the prisoners in the basement of the Town Hall, accidentally setting it on fire in the process. They know they must look for Mallecyne, but decide their first order of business must be to locate a doctor for the suffering prisoners, some of whom are in terrible shape.
Mallecyne awakens in an enormous fortress built of ice. Embedded in the walls are more of the insidious blue glass statuettes, evidently to keep her too weak to use her magic. She manages to smash them and escape from the fortress with the help of one of the maidservants. All the servants in the ice fortress have been brought there by Tynedale, and cannot leave because the place is situated somewhere near the North Pole, artificially heated by his magic -- if they stepped outside they would freeze.
Mallecyne warns the remaining servants to flee the ice fortress while they can, then she destroys it. She manages to transport herself and the maid, Chastity, to the place where she can detect the strongest portal emanations. They arrive in Boston, and eventually find their way to Porterville.
Between them, Mallecyne and Hero get the lost sorcery students back through the Temple of Demeter portal, but not before another confrontation with Tynedale and a group of louts from Porterville who decide they want the female students as their brides. Along with them go Chastity, a doctor, and the former mayor of Porterville. The last is a young and handsome man who has fallen in love with Guinevere and volunteered to be the ambassador from his world to the World Parliament on the other side.
Guinevere and Mayne take Nathaniel to the World Parliament while Mallecyne and Hero go back to Isis City to make their report on the portals. Mallecyne has managed to place a spell which effectively prevents Tynedale from using the portals himself, trapping him on the other side. But they still haven't discovered how to close the portals -- the one in Porterville still functions even though the building around it burned down.
Mallecyne, Hero and their boss, Minerva Cerridwen, travel to Valhalla (Stockholm) to attend the World Parliament. The House Speaker, Ambrosia Horus Scota, is quite hostile to them all, particularly Minerva, her estranged mother. As the former lover of Alain Tynedale (and still in love with him) she also has bad feelings toward Mallecyne, whom she believes spirited Tynedale away from her.
After Nathaniel speaks at the Parliament, they recess to discuss what should be done about the portals. He makes application to remain with Guinevere and become her consort. Ambrosia goes to Hibernia (Spain) a strange country where the borders have been sealed to prevent any of the natives leaving. A thousand years before they opted to practice the religion of Christianity, a religion that never caught on in this world due to its' barbaric and anti-woman teachings and rhetoric. When they refused to give up this religion, to practice birth control and equality between the sexes, the borders were sealed and the people left to starve or survive on their own with no trade, no educational or medical resources.
Ambrosia goes through a portal in Hibernia to search for Tynedale, reasoning that if he is in need and she can help him, they will become lovers again. Mallecyne, Hero, Guinevere and Mayne take a group of the rescued students through the Hibernian portal to find Ambrosia before the witch-hunters get her, only to discover they have landed in the same world, but a different time period. They now believe Tynedale somehow rigged each portal to go to a different historical period when the witch scare was rampant; this one was during the Spanish Inquisition.
They find another group of students who are being held prisoner in a mountain cave above a village. They are able to rescue the students, but they require immediate medical attention. Guinevere and Mayne return to their own world, recruit a medic from Hibernia (where the medical practices aren't much better than the other dimension) and take him through the portal with them.
The medic, whose name is Augustine, has a high price for his services -- he will only accompany them through the portal and help their wounded if they will magically transport him out of Hibernia (due to the nature of their religion, Hibernians have no powers) and into the outside world. They agree, and are able to get the wounded students back to Isis City. Hero, Mallecyne, and the students who originally accompanied them remain behind in Spain and go searching for Ambrosia.
They end up at a castle over the mountain from the village, where Mallecyne uses her powers to 'freeze' the inhabitants for several hours, all except for one young man who speaks English (International) to them, and becomes their unwilling guide on the search for Ambrosia.
Back in Isis City, Guinevere and Nathaniel (who became lovers during the World Parliament in Valhalla) discover that their refugee medic has his own agenda. Still an advocate for Christianity, he wants to go to the World Parliament to plead his case and that of his people. Guin wants to return him to Hibernia, but Minerva Cerridwen thinks perhaps he should be allowed to speak -- her solution to the people in Hibernia is to allow them to go through one of the portals, and live in a world where everyone has the same strange beliefs they do. Guinevere and Mayne are of the opinion this would be a rather devastating influx of people into the other world, and Minerva wants to know why they should have consideration for those who have been murdering them.
Mallecyne and Hero find Ambrosia with the help of the young nobleman. She is quite weak and has been mistreated horribly, but they manage to get her out of the castle, where a Grand Inquisitor and his men are waiting for them. Fortunately, with several of the Wizardsbane family in their midst, the blue statuettes, known as Tanterines, can be easily destroyed, and Mallecyne is able to trounce the Inquisitor and his men.
Once back in Isis City, Mallecyne and Hero plan to buy betrothal bracelets and pledge their bond formally. But before they have a chance to do so they are assigned to escort Augustine to Valhalla, where he apparently believes he will gain the release of his people from their quarantine. This isn't the case, however -- instead, the World Parliament offer him the option of taking his people through the portal into the other dimension.
Nathaniel accompanies Mallecyne and Hero to Valhalla, to begin his new duties as an Ambassador. While they are there, Guinevere and Mayne are sent to Londinium, where another portal has been discovered in a nightclub in Soho. When they arrive with their team, they are accompanied through the portal by several Londiniers, whose friends have crossed to the other side.
Expecting another battle, Guin and Mayne are amazed to find themselves in the future of the parallel dimension. The people of the other world have been led down a different path by the doctor and maidservant who crossed into Carthage with Hero and Mallecyne, then returned to their own world to lead a religious and political rebellion. Now the other-worlders embrace a multi-religious culture, but still have no magic. They have made enormous technological and social advances, however.
When Mallecyne and Hero discover that Guin and Mayne have gone to Londinium, they set out to help them. They arrive in the other world to find a banquet waiting for them, and Alain Tynedale, now nothing more than a brain in an android body, imprisoned in some kind of a glass chamber. The advances of their culture enabled the other-worlders to finally capture him, and stop his destruction of their world.
Tynedale tries to make a deal with Mallecyne, to free him if he will tell her how to close the portals. In a final confrontation she is forced to kill him. Then Mallecyne, Hero, Guinevere and Nathaniel go to Maeve's Land (Ireland) to search the spell repository to find the spell Tynedale used to open the portals. Hero locates something that is close, and Mallecyne realized he coupled this spell with another, simpler incantation, in order to open the portals. They take the physical copy of the spell to Hibernia and throw it through the portal. But upon arriving in Hibernia they discover a lot of people have come through from the other side and are planning some kind of rebellion which will free the Hibernians from the border spell that entraps them. Hero tells these people to cross through into their own world or be trapped forever on this side, and in the ensuing panic a girl is left behind. Hero is supporting her to keep her from being trampled, and when Mallecyne transports them home to Isis City the girl, Maria, is carried with them.
Since Maria cannot return to her own world, she joins forces with Nathaniel to write the history of her world. She falls in love with Mayne and he with her.
The Londinium portal never closes, and becomes a sort of gateway between the worlds, where people go for holidays and visits.
Mallecyne and Hero return home to Isis City, and finally pledge their troth, as do Guinevere and Nathaniel. Nathaniel, as the Ambassador from the other world, will now travel regularly between the worlds.