A séance is being held, as a sociable diversion rather than a serious experiment in spiritualism, at the house of South-American merchant, Montague Faull. Two unknowns, Maskull and Nightspore, arrive at the invitation of one of the lady guests. The medium, Backhouse, materialises a silent but animate "phantom body" for them all to examine, but almost immediately a further unexpected guest, Krag, bursts into the room and twists the spirit's neck right round, leaving it dead and wearing a "vulgar, sordid, bestial grin, which cast a cold shadow of moral nastiness into every heart".
Outside in the street, this uncouth stranger, Krag, who knows Nightspore, asks Maskull if he would, to see more such strange sights, journey to Tormance, an inhabited planet of the binary star Arcturus. Unsure whether Krag is joking or not, Maskull says that, if the offer were a genuine one, "For four-and-twenty hours in that Arcturian planet, I would give my life." Krag tells them to meet him at the old observatory at Starkness, from which they will begin their journey.
Starkness proves to be a semi-ruin. Maskull is bored of waiting for Krag, so Nightspore takes him to a high cliff from which can be heard a drum-like beat. He says that Maskull will hear it again at some point, but refuses to explain further. As Krag still hasn't turned up, Maskull decides to climb the observatory tower, from which, Nightspore has said, they will start their journey. But he finds climbing the stairs almost impossible. It's as if, with every step, gravity were increasing to fantastic levels. Exhausted, he sits down, and hears a voice say, "You are only an instrument... Nightspore is asleep now, but when he wakes you must die. You will go, but he will return." Maskull leaves the tower.
Back on the ground, Krag has turned up. He says that, climbing the steps, Maskull experienced the gravity of the planet they are soon to visit. As they must get to the top of the tower to make their journey, Krag makes a "careless and almost savage" cut with a pocket knife in Maskull's arm, then spits on it. After this, the mysterious weight does not trouble him, and they ascend the tower to find the "torpedo of crystal which was to convey them through the whole breadth of visible space". They enter it. Krag pulls a lever and the torpedo starts to lift. He then adjusts a "speed-valve" and, experiencing a "velocity more nearly approaching that of thought than of light", Maskull is overcome by drowsiness...
He wakes upon a vast plain, prostrated once more by overpowering gravity. He discovers that he has sprouted additional sensory organs: a third-eye-like "fleshy protuberance, the size of a small plum" on his forehead, and a "knob on each side of his neck"; while, "from the region of his heart, a tentacle had budded." He is, then, on Tormance; but he is alone, naked, and helpless.
A woman appears. Her skin is opalescent with ever-changing pastel colours, and she has the same additional sense-organs as Maskull. Her name is Joiwind, and she proves to have a kind and selfless nature. She gives Maskull clothes to wear, and then, realising how he is suffering under Tormantic gravity, performs a blood transfusion, enabling him to get to his feet at last.
Joiwind proceeds to introduce him to this world, while the two of them journey across its landscape to the home she shares with her husband, Panawe. Tormance is a relatively new world; its God is called Shaping, or Crystalman, or Surtur (a name Maskull heard from Krag's lips), and this being is regarded by Joiwind as wholly benevolent, a "friend and lover". They witness bizarre sights as they walk: a patch of migrating purple grass; a plant that is rooted in the air; a three-legged creature that moves by rotating itself; a time-mirage of the pillars of a temple that has yet to exist. This world has new colours Maskull has never seen before, caused by there being two suns instead of one. He and Joiwind drink "gnawl-water", which provides all the sustenance they need (Joiwind finds the idea of eating living creatures detestable). Panawe comes to meet them, and the trio continue towards the couple's home, at one point walking across a lake; water, on Tormance, as well as offering sustenance, is much more solid than that on Earth.
A short while later, Maskull is gripped by a "restless and noble" feeling, which Panawe explains as being due to the influence of Tormance's second sun. Up to this point, Maskull has only felt the heat (and nature) of the one, Branchspell; the distant light of the second, Alppain, is drawing him on. Panawe warns him that to stand in the light of both suns, and to experience their different natures simultaneously, may prove to be an agony more than he can bear.
Maskull begins his journey across Tormance. It is to take him through a variety of lands, and bring him into contact with a variety of peoples and creatures. There is, however, a constant, dual thread drawing him ever on. Maskull begins to learn, for instance, that Joiwind's idea of this world's God, Crystalman, cannot be true. Almost everywhere, he encounters a thing called the "Crystalman grin" — a mocking, disfiguring leer that appears on the corpses that soon begin to litter his path, something that seems to imply that everything he has gone through is some sort of joke, a pitiable self-delusion, and that this pleases Crystalman. In part, then, Maskull's quest is for the truth behind who or what Crystalman really is. Another thing drawing him on, though, is a series of visionary experiences, most often of the spectral "Muspel-light" that seems to be shining from another existence altogether; these visions are often accompanied by the drum-tapping sound Nightspore introduced him to in Starkness.
Between these two forces, Maskull is led through a series of lands, each of which has a peculiar character, a particular outlook or philosophy of its own. Maskull fully partakes of each land's nature, often changing physically to gain the particular variety of sense-organs peculiar to that region, and which reflects its people's outlook. (For instance, the chest-tentacle Maskull found himself to have grown when he first woke on Tormance, and which Joiwind explained to him as being an organ by which "what we love already we love more, and what we don't love at all we begin to love", becomes, in the violent lands of the Ifdawn Marest, a muscular third arm.)
Leaving Joiwind and Panawe, Maskull encounters a being that announces itself to be Surtur, and which claims to have brought Maskull here so as to serve it. Shortly afterwards, Maskull encounters Oceaxe, an aggressive, passionate woman who takes him to her homeland of the Ifdawn Marest, where life is a constant battle of subjugation, domination and even absorption through the sheer force of will. So soon after leaving the tranquility of Joiwind and Panawe's household, Maskull finds his hands seriously bloodied.
He leaves the Ifdawn Marest in the company of a different woman, Tydomin, at first intent on letting her take his life for the crimes he has committed. This all changes when, on the verge of death, he finds himself looking out of the eyes of the "supernatural youth" which Backhouse, the medium, materialised at the séance on Earth. Only, now, Krag twists his neck, and "kills" him. Maskull comes back to life on Tormance with a renewed sense of purpose, and journeys on.
He next comes across a man called Spadevil, at first seen striding amidst a veritable forest of static lightning bolts. Spadevil has exiled himself from his homeland of Sant, but is about to return, taking with him his new doctrine: that of living life according to nothing but one's duty. Maskull and Tydomin accept this idea and accompany him, but Spadevil is not welcomed in Sant. Maskull, in fact, becomes the instrument of Spadevil's death, when an elder of that land, Catice, destroys one of the sense organs Spadevil altered on Maskull, and Maskull, his perceptions changed, recants Spadevil's new philosophy.
With yet more carnage piling up behind him, Maskull enters the Wombflash Forest, and sees a vision of his own death at the hands of Krag. He then meets a fisherman, Polecrab, who seems content in his existence with wife and children. His wife, however, is drawn by the eerie music floating over the seas each night from Swaylone's Isle. It is said that listening to this music drives people first to madness then death. Maskull, characteristically, decides to pay the island a visit. He survives hearing the music played on Irontick (a sort of lake of liquid metal), and elects to play it himself. Attempting to summon a vision of Muspel, he ends up destroying the instrument, and finds that he has killed the musician Earthrid, too.
Maskull moves on. He crosses a violent sea to the river valley of Matterplay, whose waters are so imbued with vital energy that a hundred different forms of life pop into existence every moment. Beyond it, he falls in with a Phaen (a non-human creature that is neither male nor female, but of a third sex, which seeks fulfilment not through union with another of its species, but through finding its god, Faceny), and helps it get through to the underground realm of Threal where, the Phaen is convinced, it will find Faceny. Instead, it immediately drops dead.
In Threal, Maskull encounters Corpang, a religious man, who takes Maskull to the three idols that are worshipped in that land. As Maskull stands before them, the idols gain the disfiguring Crystalman grin. Corpang is disillusioned, and the two leave Threal for the next domain on Maskull's journey: Lichstorm.
In Lichstorm, relations between the sexes have been reduced to a tormented parody. Men are only ever brutal and ugly, women are almost formless beings exuding a siren-call of sexual sweetness; men are driven to find them by an almost painful call that only ever ends in death. Maskull and Corpang join Haunte, a man of Lichstorm, on his quest for the woman Sullenbode. Haunte is killed when he finds her, but Maskull manages to transform her into an apparently fully human woman. She joins him and Corpang as they continue their quest for Muspel, though Corpang objects to the presence of a female, and eventually sets off on his own. Sullenbode says that she can only live as long as she has Maskull's love; Maskull is willing to stay with her, but is overcome by a vision of Muspel-light. Distracted, he follows it, then wakes from its influence to find Sullenbode dead at his feet. He buries her and journeys on.
Beyond Lichstorm, he enters the watery land of Barey. He is heading onwards to the ocean, where he will be able to fully behold the rays of Tormance's second sun, Alppain. On the way he meets Krag, who is quite unapologetic about abandoning him on his own on this strange world, and soon after the pair meet up with a third man, Gangnet, whose sympathies for Maskull's sufferings on this world provokes Krag's most vehement sarcasm. It soon becomes apparent that Gangnet is Crystalman, that Krag is a being of equal potency, and that the two are old adversaries. Krag says that Maskull is soon going to die.
They come to the ocean and set out on it. Gangnet preaches to Maskull of self-abnegation as a way to reconcile him to his coming death; Krag is contemptuous. Maskull has a vision of Krag beating the drum-taps he has heard so many times; Krag says he is beating them on Maskull's heart.
They enter a foggy region. Gangnet is burned by the light there — it is Muspel-light — and flees, having revealed his (Crystalman's) true face to be "vulgar, slobbering". Krag says that the ocean is called Surtur's Ocean. "Where's Nightspore?" Maskull asks. Krag replies, "You are Nightspore."
They pass through the fog to a small island with a tower on it. This, Krag says, is Muspel. Nightspore climbs the stairs of the tower and, through the windows as he climbs, sees a series of visions of Crystalman's world as it appears from the outside. What these visions reveal is that Crystalman is a malign being feeding off the deaths of human souls, and who, to ensure his supply of such deaths, lures, with the promise of pleasure, these souls from their true home, which is Muspel.
Muspel, then, has been reduced to a tiny, beleaguered island, menaced by the all-surrounding, all-devouring Crystalman. But only Krag, and now Nightspore, know this truth. The book ends with Nightspore vowing to return to Crystalman's world — to our Earth — so as to continue the fight against their adversary.