In 1865, Rudi Matt, James MacArthur works as a dishwasher in Kurtal’s hotel kitchen and nourishes a daydream of reaching the Citadel’s summit in the Swiss Alps. Old Teo Zurbriggen, Laurence Naismith, the hotel’s cook and a former mountain guide, scolds him after he knocks over several dishes. Undeterred, Rudi slips away to fetch his hidden climbing gear and slips into the surrounding mountains. Lizbeth Hempel, Janet Munro, the hotel proprietor’s daughter who is drawn to him, encourages his boldness, and Teo covers for his absence.
On the glacier, Rudi hears a cry for help and rescues Captain John Winter, Michael Rennie, a British mountaineer who has fallen into a crevasse. On the return to Kurtal, the two men discuss the climbing accident on the Citadel that led to the death of Rudi’s father, who saved his client by wrapping him in his red shirt. Rudi, ashamed, reveals that he is a dishwasher and asks Winter not to tell anyone that he was climbing.
Back in Kurtal, Klaus Wesselhoft, Lee Patterson, vies for Lizbeth’s attention, while she remains focused on Rudi. Franz Lerner, James Donald, a mountaineering guide and Rudi’s uncle, drops by the kitchen and learns that Rudi has run away. He is furious at both Rudi’s audacity and Teo’s cover-up. That evening, Franz tells Rudi that he knows about his activities on the glacier, and Franz and Ilse, Rudi’s mother, discuss the future they have in mind for him. Captain Winter arrives and asks if Franz is available for a climb on the Wunderhorn the next day. Franz mentions that they will need a porter, and Winter suggests Rudi. Franz is initially against it, but relents when Winter reveals that Rudi saved his life earlier.
During the climb, Winter studies the face of the Citadel and discusses possible routes with Franz, though Franz is not interested. As Winter takes notes and Franz naps, Rudi sneaks away to search for a better route back down, but becomes trapped and has to be rescued by Franz and Winter. The next day, Winter prepares to leave Kurtal for a time and tells Franz he would like to climb the Citadel with him and asks him not to be too hard on Rudi. While preparing for work at the hotel, Rudi discovers that the boots and climbing gear gifted to him by Winter have been taken by his uncle, but then encounters Lizbeth, who has bought the boots from Franz.
The next day, Teo takes Rudi and Lizbeth to the Felsberg to teach Rudi about how to be a guide. Back at the kitchen, Rudi discovers that Captain Winter has left Kurtal for good after his plans with Franz didn’t work out. In the tavern, Franz rants about Winter’s foolishness. At the encouragement of Teo and Lizbeth, Rudi asks Franz for another chance to join him as a porter. Franz, drunk, refuses and humiliates him in front of all the men of the village. During the choral festival the next night, Rudi overhears the men of the village talking about seeing a climbing party camped on the slopes of the Citadel. Rudi leaves the village to join the climbers. In the hut, he again meets Captain Winter, who is climbing with Emil Saxo, a guide from the rival village of Broli, [Herbert Lom]. Rudi asks to join Winter’s party, lying to him and saying that he had been given permission by Franz. Winter agrees to take Rudi with them, and they make a series of climbs to reconnoiter routes to the top.
In Kurtal, Teo and Lizbeth watch the face of the Citadel through a telescope and see Rudi with the climbing party. Teo goes into the tavern and shames the men of the village for their cowardice and reveals that Rudi is the third man on the mountain. Franz, incensed, says that he will go up to the Citadel to retrieve Rudi and talk Captain Winter and Saxo out of the climb. Other men of the village, including Teo, join him. That evening, Winter and Saxo leave to get supplies in Broli. As they depart, Winter, believing Franz has given Rudi permission to be there, sends Rudi back to Kurtal to ask Franz once more to join them. Instead, Rudi climbs further up the mountain trying to discover his father’s route to the top while the weather deteriorates.
Winter and Saxo return to the base camp and meet Franz, who reveals that Rudi did not have his permission to climb. As they talk, Rudi, exhausted from having been on the mountain all night, enters the hut and exclaims that he has found his father’s route up the mountain. Teo argues that Rudi should have his chance to climb, while Winter tries to convince Franz again to join the climbing party. When Saxo accuses the Kurtal men of cowardice, Franz argues that a guide from Kurtal would never leave his client while going on alone. Early the next morning, Saxo leaves alone, while Rudi, having overheard the conversation the previous night, sneaks out of camp to follow him.
Back in Kurtal, Ilse discovers that Rudi is on the mountain and is furious that the village men have lied to her. She blames Lizbeth for Rudi’s obsession with the mountains, but Lizbeth tells her that Rudi must do what makes him happy. Ilse, though saddened, understands the truth of what Lizbeth says. Winter and Franz awaken and go after Rudi, who is racing Saxo to the top of the mountain. Saxo falls and injures himself badly. Rudi sees him fall and climbs down to help him. Saxo derisively tells Rudi to leave and claim his prize and become “his father’s son,” which prompts Rudi to instead tend to Saxo’s injuries and help him down the mountain. Franz and Winter encounter Rudi’s pack but do not find Rudi and Saxo. Pressing on, they carry Rudi’s pack—with his father’s red shirt—and climb to the summit, while Rudi helps Saxo safely back to their base camp.
The villagers of Kurtal celebrate the climbers’ conquest of the Citadel, but Winter refuses credit and says the victory belongs to Rudi and his father, then invites Rudi to look through the telescope, where he sees that his father’s red shirt has been planted at the top of the fortress.
