A mysterious group watches the X-Men fight Ulf, the Ice Troll, whom they had sent out as bait so they could learn more about Iceman.
Ulf attacks a fishing boat, and just when the X-Men thought they had the situation under control, Iceman finds that he is unable to freeze the ice troll. That is when the mysterious group shows up, tranquilising Cyclops, Marvel Girl, Beast and Angel, and kidnapping Iceman.
When the X-Men come to, Iceman has disappeared, along with Ulf and the mysterious group that ambushed them. Adding to the mystery, a blond man claiming to be a doctor shows up trying to gather information on the mysterious group that the X-Men had battled. The man had been tracking them for awhile.
Xavier is finally able to telepathically contact his X-Men, who promptly give him a rundown of the fight that took place and Iceman’s capture. Xavier senses a stranger among the X-Men who seems to be more than the human facade he displays.
While Angel scouts for a good landing spot for Xavier’s jet, Cyclops is keen to hear what the man knows about all this. The man reveals that the mysterious group is called the Vanir, and if they figured out his true identity, they “could go into hiding indefinitely”. With that tip, Xavier thinks he has the strange man pinned and tells the X-Men that he is “an ally [they] want”.
Meanwhile, Iceman is being prepared for the Vanirs’ sacrifice. Iceman wakes up and finds himself chained to a sacrificial altar, and wearing a rune rendering him powerless as it draws from his ice powers.
The X-Men arrive at the Vanirs’ icy grounds. Xavier remains in the X-Jet to supervise while the rest advance on foot. They see Iceman being brought into an ice cave by Ulf for the sacrifice and act quickly to rescue their team mate. Iceman narrowly escapes the jaws of death while Ulf is sacrificed instead to awaken “Ymir”.
Ymir towers over everyone and proceeds to destroy anyone who is in his path. The strange blond man who has travelled with the X-Men reveals himself to be Thor of the Avengers. Even with the aid of the X-Men, Thor finds Ymir to be a challenge to bring down and becomes frozen in the process.
It is now up to the X-Men to take down Ymir. While the rest are struggling, including Xavier to land the X-Jet, Iceman finds his powers thriving in the icy conditions and steps up to the challenge. He comes face to face with Ymir, who thinks that Iceman’s intention is to replace him as “a Lord of Winter”. With Ymir distracted, Cyclops sets off to free Thor.
Thor summons his powers and banishes Ymir to “a nether realm of Asgard”. He warns the X-Men that Ymir has become stronger since consuming the rune and Ulf and may return. Thor thanks and bids the X-Men farewell as he sets off to warn the Avengers of Ymir.
Before taking his leave, Thor has a one-on-one with Iceman, saying that according to legend, Ymir was once a human with powers like Iceman’s. It is Iceman’s responsibility to use his powers well and for good and not become another Ymir.
But elsewhere, the retreated Vanir continues to spy on Iceman, and considers Thor’s words that they could very well create a “new frost giant” out of him.