There was no correlation for the Rhinemaidens (or Rhine Daughters) with any particular saga; it seems purely an invention of Wagner, based on stories of water fairies, who were often mischievous. The Rhinemaidens frolicked in the Rhine, while guarding a lump of enchanted gold for their father.
[Quibble: The father, for whom the Rhine Maidens were guarding the gold, is unspecified and unexplained, possibly representing a power that preceded the gods themselves. It is clear that this father is NOT Wotan. While the gold often ends up in the Rhine, it is only with Wagner that it originates from the Rhine, instead of from the Nibelungs.]

The Nibelung Alberich, who somehow appeared from the subterranean depths, coveted the gold, but he was told that it was useless unless he could foreswear love.
At Seattle Opera, these scenes were done through an obscuring curtain, with the Rhinemaidens on trapezes.
Somehow, Alberich was able to swim without drowning, and he finally was able to steal the gold. This was a total surprise to the Rhinemaidens, who wondered how an obvious flirt could manage this. Alberich plunged into the depths, heading back to Nibelheim, leaving darkened waters behind him.
Back in Nibelheim, Alberich was able to transform the gold into the Ring of Power.
[Quibble: How was Alberich able to convince the powers that be that he had actually foresworn love? Perhaps it was because he had so few social graces, that he could never have gotten a date anyway?]
Freia was the goddess of fertility, and Wotan's sister-in-law. One of her chief jobs was to maintain the tree that produced the Golden Apples, that gave the gods their immortality.
This fact seemed to have been forgotten, when Loge, acting for Wotan, promised Freia to the Giants, as payment for constructing Wotan's fortress, Valhalla. Since Wotan was quite sexist, he had excluded women from all the bargaining.
Wotan was essentially bankrupt, so this was a delaying tactic while he sought alternate funding. Again, Loge's advice was nearly always bad advice.
The Giants, the brothers Fasol and Fafner, demanded their payment, so they proceeded to take Freia. For them, there was nothing around more valuable and desirable than Freia, and they were not interested in any intellectual arguments; they were only interested in payment for their services.
While the Giants were not particularly powerful, and while Wotan had subdued their race, he could not simply use force against them, because of those pesky runes on his spear.

Without Freia tending her apples, the gods began to age. Loge reminded Wotan about the story of the Ring, and suggested that Wotan take the Ring from Alberich by force, since Alberich had stolen it in the first place.
When the gold was offered to the Giants as an alternative payment, they became very interested, since they were jealous of the Nibelung's power. They agreed to consider the gold, but only after they had seen it.
Wotan and Loge entered into the depths, as they headed toward Nibelheim, the world of the Nibelungs.
[Quibble: Why did Wotan want the Ring anyway, since he is the character least likely to be able to foreswear love?]

Deep in a cavern in Nibelheim, Alberich had forced his brother, Mime, to create the magical head piece, the Tarnhelm (magic helmet,) that allowed the wearer to change shape or become invisible. I assume that this was through the power of the Ring.
Alberich tested the Tarnhelm by using it to turn himself invisible, and then proceeded to terrorize Mime. Alberich discovered that he could now also read Mime's mind, and he slapped his brother for having thoughts of possessing the Ring; then he left his brother alone, while he attended to other business.
The power of the Ring had driven Alberich to become a tyrant, literally enslaving his fellow Nibelungs, as he forced them to mine a hoard of gold for him. Alberich had mastered the Ring, but he had become extremely uncivil, and his fellow Nibelungs despised him.
Meanwhile, Wotan and Loge had arrived, and Loge immediately started to work with the disgruntled Mime, to find a way to get the ring from Alberich.

Alberich returned and discovered Wotan and Loge as intruders, and he remembered his dislike of dealing with Loge.
Wotan and Loge used cunning to banter with Alberich, while they searched for weakness in the Nibelung's defenses.They urged Alberich to display his powers; Alberich transformed himself into a serpent.
Finally, Wotan tricked Alberich into transforming himself into a small gray toad, at which point Wotan captured the toad.

Wotan and Loge returned to their surface glade in the forest, across a valley from Valhalla, with a very angry and distraught Alberich in their grip. Wotan made Alberich force his Nibelung workers to deliver the hoard of gold. Finally, Wotan demanded the Ring, as the final payment for Alberich's freedom.
Alberich was set free, but he proceeded to put a curse upon the Ring.
[Quibble: If he no longer had the Ring, how could he do this?]
This was the curse that set the stage for the rest of the operas.
Wotan proceeded to bargain with the Giants, offering to exchange the gold for Freia. The Giants wanted the pile of gold to be so huge that it obscured their view of Freia, but there was a small hole in the pile.
The Ring could have plugged the hole, but Wotan did not want to part it.

Erda, the goddess of the Earth, suddenly arose from the ground next to Wotan. She warned Wotan that he must part with the Ring, for she had seen the destruction of the Gods, as the curse played out.
Reluctantly, Wotan handed the Ring to the Giants. Freia became a free woman.
Immediately, Fasol and Fafner began to fight over the booty they had collected. Loge urged that Fafner be given possession of the Ring, so that he could guard its powers. Fasolt grabbed the Ring, then Fafner killed his brother with one blow from his staff.
So began the curse of the Ring, as Fafner left with the gold, the Ring, and the Tarnhelm.
Fafner would use the Tarnhelm to transform himself into a dragon, in which form he would lazily watch over all the treasure.
[Quibble: This seems like a strange and uninspired end choice of transformation for Fafner, so it must be another of these mythology things. Or maybe, as a child, he actually dreamed of being a dragon some day, in which case he probably needed therapy.]

As soon as Fafner had left, the gods rejoiced that the dilemma had been resolved; of course, they had overlooked the curse of the Ring.
Wotan's son Donner (=Thor) cleared the air, allowing the rainbow bridge to form.
Loge decided to return to pure flame, not to be personified again in the other operas.
Triumphantly, the gods entered Valhalla via the rainbow bridge, while far below, the Rhinemaidens lamented the loss of their treasure.
[Quibble: Loge looked and dressed too much like the other gods in this production. Other productions have him looking much more sinister and fiendish.]

 

Das Rheingold Libretto

Die Walküre