Thor has gone dangerously mad (since #460). Thanos has him restrained in a force box. He and Dr Strange, Silver Surfer and the Infinity Watch (Adam Warlock, Drax, Gamora, Moondragon, Pip The Troll and their ally Maxam) have brought the Thunder God before his father Odin who tried to mentally cure him in Warlock & Infinity Watch #25 but failed. When Thor breaks free of the containment he (with the Power Gem on his brow since #469) will be a danger to Asgard and the universe. So Odin has decreed that he must die.
Thor's fiancée Sif begs him to find another solution. Dr Strange agrees and Warlock suggests 1. Odin tried to remove the madness by mental force, but perhaps a subtler attempt to *free* Thor's sanity might work. So the All-Father enters his son's mind again ...
... and finds himself in a wilderness landscape dominated by a large mountain peak. He scales this and at the top sees Thor held down by shackles. The Thunder God says he is too weak to break free but Odin says it is the chains that are strong because *he* created them by tampering with his son's mind and making him share his being and power with others (Donald Blake, Eric Masterson and Beta Ray Bill), thus fracturing his ego. He begs Thor's forgiveness and promises never to do it again. And then he breaks the chains that bind him.
But Thor is worried that the Dark Valkyrie will return. His father assures him that she is but a figment of his imagination birthed from his madness. But she *does* appear to challenge the All-Father's supremacy. She claims that she has grown from a mere manifestation of Thor's illness, urging him on to vengeance against his father and all who stand with him. She became real (from #469, part 5 of the Blood & Thunder crossover, to part 11 Warlock Chronicles #8) and intends to be so again. Meanwhile in this mental realm she is in control, and she proves it by becoming bigger than the mountain. She dares Odin to fight her because killing her might damage his son's mind irrevocably.
Odin grows large too and the 2 titans engage in combat, his 'trident' against her sword. Meanwhile Val taunts him with the abuse he has admitted to inflicting on his son. Odin replies that he will make amends for that by freeing Thor from her influence. She counters that *she* has freed Thor from *his* influence.
Meanwhile Thor himself knows that his father holds back for fear of hurting him. And that he must take that responsibility upon *himself*. So he sends lightning to part the combatants and attacks Val himself. He would rather die than continue to be her slave. She counters that he loves her, but he says he was blind but now he sees clearly. His father cautions him to be careful but the son risks all to destroy or banish her. He strikes a mighty blow with Mjolnir ...
... and finds himself back in Asgard with Odin and the others. Valkyrie is gone and he feels whole again. Sif rushes to his side and he thanks her and Beta Ray Bill for their attempts to bring him back from madness. He also thanks the band of heroes and gives the Power Gem back to Drax, but Adam takes it until they can think of a safer place to keep it (than in Drax's stomach). Thor also reluctantly thanks Thanos but asks the 'villain' to leave Asgard and never return. The Titan says he wouldn't have it any other way. However the heroes are welcome in Asgard any time but for now he sends them home leaving him with Odin, Sif and Bill.
But down in a cavern Loki in chains (since #455) swears to be free to trouble his brother once more.