The Jotuns want help
The Avengers are called out to destroy the monster of the week, and through a magic anomaly (I leave it to you to decide how), Loki shows up (wielding a weapon similar to the scepter from the first avengers movie) dressed in furs and leather. Everybody is like 'what the hell', thinking they are going to have another enemy to fight. Instead Loki casts a single, or a few, spells and eliminates the threat. Tony and Steve try to question Loki, who is a jotun btw, when Thor just charges him. He tells the avengers that it's a jotun and how he's more dangerous than anything they, as mortals, have faced.
Loki, recovering from the blow, makes a comment about 'the giant slayer' being on midgard. Thor, realizing Loki's size starts to insult him for being a runt. This sets Loki off and he repeatedly cast spells but none of them are causing any real damage.
So Thor and Loki go at it for a bit before Steve intercedes, demanding to know why Loki is there.
It is here Loki reveals he is in need of help, and not for himself, but for the whole of Jotunheim which is on the brink of death.
They take this back to the tower where much banter occurs between Loki and Thor while Loki is trying to relay the seriousness of his world's situation. Thor remarks on the incompetence of a jotun runt and how desperate the jotuns must be to send such a weakling. Somehow or another Asgard's armies have been all but decimated giving Loki enough confidence to reveal how valued he is (for further details referr to chapter 3), and how they no longer fear Asgard's wrath. Thor, of course, is livid by this point, when Steve asks how they could possibly help.
Loki had felt a powerful sorcerer was on Midgard and had planned to ask their assistance, but knowing Thor was there, would enlist his aid instead. Thor refuses and Loki gives him a choice of helping him or Loki claiming a blood right for killing Laughy and crippling one of his brothers (the " run home, princess" jotun from Thor) by now another Small jotun joins them, Angborda-Loki's teacher. Much happens between this point and Thor's reluctant assistance, including the appearance of another small jotun, Jarsaxa - Loki's in-law. Loki reveals that the casket of ancient winters was created on Midgard and with a tremendous amount of magic, but since there are so few of the jotuns left and the deteriorating conditions of their home they don't have enough magic to perform the ritual. So Thor was going to be a conduit for them to channel the magic they need.
Loki is smug that he managed to get Thor's help, but the other two don't act all that thrilled. Everyone assumes it's because of Thor being Asir.
When the jotuns and Thor leave to perform the ritual another jotun shows up, this one blind, unsteady on his feet and normal sized, demanding to know where Loki is, and saying they have to stop the ritual. The team asks him why he doesn't want the ritual to happen since it is what will save their people. The jotun reveals the casket requires a sacrificial heart from a powerful jotun sorcerer, and that Loki intends to kill himself to create a new casket. Everyone tries to get to the location and...............