It all starts in the far North with the god Baldr.
Baldr was the fairest and kindest of the Norse gods, beloved by just about everyone, and the maker of many fine and wonderful things throughout the world. He was always happy and sunny and cheerful, until he started having nightmares. Terrible dark dreams about meeting an awful fate.
Night after night, they plagued him, until his cheerfulness began to diminish and dark circles were under his eyes, and his face seemed always worried, and the other gods, and especially his mother Frigga asked him what was wrong. So, he told them about the nightmares, and everyone agreed that this seemed like some super bad juju going on, so they went to his father Odin for counsel and told him about the nightmares.
Odin agreed that this very much seemed like a Bad Thing, though he didn't have an answer yet as to what was going on. So, Odin mounted Sleipnir and rode through the world to find an answer. Nothing on earth could explain it, so finally he journeyed into the underworld to consult with a seeress there.
Normally, Hel's realm isn't a very happy place, unless you're in Valhalla, so imagine Odin's surprise when he arrived to see the place all decked out with lights and decorations and a great feast laid out in welcome. Odin was pretty sure it wasn't meant for him. He hadn't exactly called ahead to let them know about his visit. He found the seeress there and woke her and asked her what was going on, what's up with the feast, who are they expecting? And the seeress told him that it was for Baldr, who was to be the guest of honor.
Odin was very much dismayed to hear about his son's impending doom, and what she foretold matched up with Baldr's nightmares, so it was with a heavy heart that Odin returned to Asgard and told Frigga what he'd found out. Needless to say, Frigga wasn't exactly thrilled by this either.
Frigga couldn't imagine who would want to hurt Baldr. Everyone liked him, it seemed--he was easily the most popular person in all creation. But she didn't want to take any chances. So she set out on her own ride, demanding oaths from everyone and everything she could find, living and non-living alike, animal, vegetable and mineral, that they would never cause harm to Baldr. And they all gave their promise, everything and everyone everywhere, except for one tiny, baby, innocent plant: the mistletoe.
In some versions, Frigga overlooks the mistletoe, the youngest thing in creation, thinking it was too small and innocent to ever hurt Baldr so there was no point in asking. In others, the mistletoe, being so small and young, never having seen the sun as it grew in the shadow of the mighty oak and having no idea who Baldr even was, that it doesn't give a promise, wondering what the heck this weirdo lady is going on about.
Still, satisfied with her mission, Frigga returned to Asgard, confident that now her son would be safe. She showed the recorded oaths to Odin and Baldr, and everyone thinks it should all be hunky dory now. In fact, to test these promises, people started throwing things at Baldr, and they either just harmlessly bounced right off him, or missed him entirely, no matter how true the shot was. It didn't take long before Throwing Things At Baldr became the hot new popular game on Asgard.
Well, about this time, Loki, who had morphed from "playful, clever and sometimes obnoxious prankster" into Full-Fledged Jerkface Who Hates Everyone, figured he could really have some fun with this. So, putting on his best innocent face, he went to Frigga and was like, "Gee, did you really get the oaths of EVERYTHING not to hurt Baldr?"
And Frigga was like, "Yup, sure did."
"You mean everything everything?" Loki asked.
"Well, yeah," Frigga answered, "Except for the mistletoe, but it's just a baby, so nobody's worried about it."
And finally, Loki had his ammunition. Figuratively and literally.
Loki went and got himself a sprig of mistletoe, and fashioned it into a weapon. Some say a dart, some say a spear, some say an arrow, but regardless, it was now a Sharp and Pointy Thing.
Now, there was one god, Hodr, who wasn't joining in with the reindeer games of Throw Stuff At Baldr, because he was blind and therefore his aim sucked and he was just as likely to hit someone else with throwing things as he would Baldr. Loki knew this, but still went up to Hodr and was like, "Hey, buddy, don't you want to play too?"
And Hodr shrugged and was like, "I'd like to, but I'm blind. I don't exactly have hand-eye coordination here."
And Loki was like, "Aww, that's too bad. Tell you what, why don't you take this thing here, and I'll guide your hand so you can play too!" And he handed Hodr the mistletoe. Hodr, having no idea what he was holding or what harm it could do, thought this sounded like a great idea. So, off they went to play the game.
...it went exactly as well as you think it did.
With Loki guiding him, Hodr launched the mistletoe at Baldr, and sure enough, it struck true. And struck Baldr dead. And everyone FREAKED THE HECK OUT. Hodr and everyone else was horrified by the "accident" Loki had engineered. Loki did his best "surprise" face.
Frigga was like, "OK, clearly there's been a mistake here. Maybe it's not too late. Is anyone willing to go to Hel and see if we can figure out a way to fix this?" Hermod, Baldr's brother, volunteered for the job. Odin loaned him Sleipnir, and off he raced.
Hermod went up to Hel and was like, "So...... pretty sure this was a mistake. He's not supposed to be dead. Do you mind giving him back to us?"
And Hel was all, "Nope, sorry, he's mine now. He's gonna stay here FOREVER. Daddy said I could keep him." 'Daddy,' of course, being Loki himself.
And Hermod was all, "Aw, come on, lady, help a brother out." So she was like, "OK, OK. Everything and everyone promised not to hurt him? If everyone and everything is willing to weep and mourn Baldr, I'll let him go."
Hermod figured this shouldn't be too hard. Everyone loved Baldr and would miss him, right? So he reported back with Hel's condition. And all the gods mourned. Even Sunna, the sun, turned her face away, and the world became cold and dark, and leaves fell from the trees. And they went forth and told all creation what had happened, so that they would mourn too.
Everything seemed to be going according to plan, and it seemed like they'd have Baldr back in no time. And then they came across a giantess named Tock. And when they told her about Baldr's death, she just shrugged and went, "Meh." And the gods were shocked.
"Why aren't you crying and mourning?" they asked.
And Tock, who was actually Loki in disguise was like, "Didn't really know him. Didn't really care for him, all high-falutin' and being Little Mister Perfect. Why should I cry over him? His death doesn't mean anything to me." And because of her refusal to weep for Baldr, Hel's condition could not be met, and Baldr could not be saved.
When Frigga found out about this, her grief turned to rage, and if she could not have her son back, then that which killed him must be punished and wiped from the earth. No one blamed Hodr; he was blind and it was an accident, after all. But the mistletoe, well. That was another story.
Now, during all this, the poor mistletoe didn't really understand what all the fuss was about. Sure, it felt bad for the accident, but it didn't know why it warranted such grief from everyone. So, for the first time, the mistletoe went out into the world, and saw many wondrous things and when it asked, "who made this?" The answer kept coming back: "Baldr made it." And slowly the mistletoe realized the magnitude of the loss of Baldr, and it too began to weep and grieve, as deeply as anything else in all creation, unaware that the gods were hunting for it on Frigga's orders.
It was Freyja who found it first. And when she heard the little plant weeping, she asked it why it was crying so, and it wailed about how sorry it was and how it too grieved the loss of Baldr and all the light and love he had been. So, Freyja asked if it would be willing to pay a wergild, a blood-price, to Baldr's grieving family, and the mistletoe promised it would.
So, Freyja gently brought it before Frigga, who demanded to know why the mistletoe was still alive when Baldr and Baldr's wife Nanna (who had also died in her grief over him) were not. Freyja explained that the mistletoe wished to offer a wergild in an effort to repay for Baldr's life, if only Frigga would accept it. The mistletoe, so moved by what it had learned about Baldr, and by Freyja's kindness to it, offered that, in honor of the love lost, and the love it had been shown, that it, too, would strive to kindle love. Would that not have been what Baldr would have wanted? To be memorialized in love, after all? And so, from that day forward, all who would meet beneath the mistletoe would come together in peace and love and share a kiss. Frigga accepted this wergild, weeping tears of gratitude that became the plant's white berries, and Freyja herself accepted the mistletoe's allegiance.
And that is the story of the mistletoe, and why enemies who meet under it must be in peace, and why lovers kiss beneath it.