Matt Cruse is piloting an aerocrane for the France's Celestial Tower where he narrowly survives a terrorist attack by the Babelites, a group of people who are opposed to humans reaching the heavens. After the incident, he meets with Kate de Vries, and is saddened to hear that Kate will soon return to Lionsgate City.
Canada's Minister of Air wants Canadians to be the first in space, and invites Kate to join the expedition as an expert in aerial zoology, while Matt is offered a chance to become one of the world's first astralnauts on board the Starclimber. Matt decides to visit his mother and sisters, while he is in Lionsgate City; he accepts a party invitation sent by Mr. and Mrs. de Vries. During the party, Matt is informed that Kate's parents will probably marry Kate to James Sanderson. Upon hearing this, Matt seeks out Mr. Sanderson at the party.
Matt is enrolled in the astralnaut program and becomes friends with fellow astralnaut trainee Tobias Blanchard. Three people are chosen to be the first astralnauts, but Matt is not amongst them. When he leaves the room, Kate follows him to try and cheer him up. When she takes his hands, Matt finds an engagement ring on her finger. Matt leaves, heartbroken, and goes to a bar with the rest of the rejected astralnauts-in-training. The other astralnauts tell him that he deserved to be chosen. The next morning, Captain Walken tells him that one of the men they chose broke his leg, and offers Matt a spot on the space trip as a replacement. Matt accepts, even though the thought of seeing Kate again is painful.
On the day of the Starclimber's maiden voyage, the complete crew is assembled: astralnauts Chuck Sheperd, Tobias Blanchard, and Matt Cruse; photographer and journalist Evelyn Karr with her pet monkey Haiku; zoologists Kate de Vries and Sir Hugh Snuffler; along with Chef Vlad Herzog, Captain Samuel Walken, and Dr. Sergei Turgenev. They board an airship and are brought to the Starclimber's launch site. During the trip, Kate apologizes to Matt, telling him that the engagement was just a way to obtain her parents' consent to go on the space trip, and that she will break it off when she returns home. Matt is not comforted, especially when Kate tells him that they must be careful about whatever suspicions the crew members have about their relationship. The Starclimber is revealed to be an orbital elevator-type cabin, rather than an actual "ship", and climbs on a cable that is held up by a rocket that acts as the counterweight.
When the crew is at zero gravity, Tobias is chosen to be the first man in space. He is enthralled by the endless vista of space in front of him and threatens to unclip his harness, provoking Matt to rescue Tobias. That evening, they discover a problem in the cable that endangers the crew. They quickly come to the conclusion that the rocket that pulled the cable up did not reach the proper altitude because of a blown fuse. The team successfully relaunches the rocket, and Matt rescues Captain Walken along the way. Later on, they discover a new creature - which Kate names "etherian" - a limbless creature with baleen that moves by ejecting bursts of air from various holes in its body and emanates light like a firefly.
During the journey, Matt discovers a letter from James Sanderson to Kate, leading him to believe that Kate really is preparing to marry James. That night, he tells Tobias his feelings for Kate, hoping that he can help him with his troubles. Tobias suggests that Matt propose to Kate. When he does, Kate attempts to change the subject, only to provoke Matt into claiming that she is lying to him about breaking off her engagement with James Sanderson.
A few days after Matt's failed proposal, the Starclimber is homebound. However, unexpected astral barnacles are discovered on the cable. The cable snaps, Shepherd is killed, and the Starclimber is left drifting in space with no way to return to the surface. The remaining members of Starclimber are left in orbit around the earth when Matt develops a crude but brilliant brainstorm using an emergency oxygen tank as a propellant and using the toilets' flush mechanisms as maneuvering jets, giving the crew only one chance to reenter the atmosphere. The beginning launch is successful, but Captain Walken is knocked unconscious, and Matt and Tobias are left as the only members of Starclimber's crew who are able to pilot her. Together with Kate and Dr. Turgenev, the four manage to guide the Starclimber into the first stage of reentry. While they are falling, Kate throws away her engagement ring and confesses her love for Matt. However, Dr. Turgenev reveals that everyone already knew, adding that it was "painfully obvious". The Starclimber, after a turbulent fall, successfully lands in Cairo.
Kate receives a telegram from her mother, saying that James Sanderson had eloped with another woman. Once Kate and Matt are alone, she apologizes to him. Matt then proposes to her again, using Sanderson's engagement ring, and she accepts, though they both know that getting Kate's parents' consent will be difficult.