Let's jump right in, shall we? Some merchants travel home to Babylon from Rome. When they return, they yammer on and on about great beauty and virtue of the Roman Emperor's daughter, Custance.
In fact, the Sultan of Babylon is so swoony over the merchants' descriptions of Custance that he decides he just has to marry her, no matter the difficulties involved in convincing a Christian to marry his daughter to a pagan. The Sultan agrees that he and all his closest allies will convert to Christianity, and—badabing—the marriage is arranged. Except that no one bothered to ask Custance, who is pretty upset about about leaving her friends, family, and culture for a pagan nation. But she's a good, dutiful princess, so she zips her lip and heads to Babylon with a bunch of fancy pants attendants in tow.
It turns out Custance isn't the only one up in arms about the impending nuptials. The Sultan's mother isn't havin' it either, mainly because she thinks converting to Christianity and marrying her son to a Christian is a terrible idea.
So she does what any reasonable mother-of-the-groom would do: she gathers her counselors and plots to kill her son, his allies, and all the Christians at a great feast she prepares for them. After the massacre, she plans to rule Babylon herself. The plan goes off without a hitch, but instead of killing Custance, the Sultanness has her cast out to sea in a rudderless boat filled with food. So at least she won't go hungry?
Custance drifts on the sea for three years and is miraculously kept alive only by God. One day, she washes up on the shores of Northumbria, and a Constable of the king finds her on the beach. He takes her into his home, where Custance converts his wife, Hermengyld, to Christianity.
Unfortunately, though, a knight living nearby gets all kinds of PO'd when Custance rejects his advances. One night when the Constable's out, he sneaks into his house and slits Hermengyld's throat, placing the knife next to Custance's hand. Good grief, the luck on this girl.
When the Constable returns and finds his wife dead, he calls King Alla to help him solve the whodunit. The knight accuses Custance of the deed, but when he swears to his story on a Gospel book, a hand from the heavens strikes him down (those hands sure are handy in a pinch). This proves Custance's innocence and the knight is executed, which, can we just say? Serves him right. King Alla pulls a Sultan and marries Custance, captivated by her beauty and virtue, and probably her perfume.
Right off the bat, Custance gets a bun in the oven. Which would be great, but unfortunately King Alla is too busy fighting a war with the Scots to be a daddy. While he is away, Custance gives birth to a boy. Only instead of sending a cigar, Alla's mom (the mom's in this story are the worst), intercepts the message to him and re-writes it to say that his wife has given birth to a demon-child and is herself a supernatural being of some sort. Apparently, she's not too happy to have such a virtuous daughter-in law.
Despite this message, King Alla orders the Constable to keep his wife and child until his return. His mother intercepts this message, too, changing it to order the Constable to cast Custance and the baby out to sea in her boat. The Constable reluctantly follows the order.
So. Custance washes up on another pagan shore. Yep. Another. There, a man tries to rape her, but Custance manages to push him overboard in the struggle, where he drowns. And Custance's boat begins drifting again, at which time Shmoop suggests she might want to invest in a pair of oars.
This time, however, she meets the boat of the Senator of Rome, who is on his way home from taking vengeance on the Sultanness of Babylon for her treachery to Custance. The Senator takes Custance and her son into his household, where she lives peacefully for a long time. Yay.
Meanwhile, King Alla has executed his mother for her treachery, which sounds about right. Distraught at the loss of his wife and child, and plagued by guilt, he heads off on a pilgrimage to Rome in penance. Convenient, no?
The Senator of Rome rides out to meet him and invites him to a feast, to which he brings Custance's son, Maurice. Struck by the child's resemblance to Custance, Alla asks the Senator about the kid. When the Senator describes the child's mother, Alla is absolutely positive that it's Custance and asks to meet her.
Cue the happy music. Alla, Custance, and Maurice are reunited. Alla explains that he had nothing to do with casting Custance out to sea, and Custance asks Alla to invite himself to dinner at the Emperor's house, where she finally reunites with her father.
After a while, Custance and Alla return to Northumbria, leaving Maurice to be Emperor of Rome after his grandfather. Before a year has passed, however, Alla dies. Custance returns to Rome, where she lives out the rest of her life in piety and almsgiving and Emperor's-mom-being.