In 1915, amid World War I, Heidi Juliette Caton still lives on the Swiss Alps with her grandfather Jan Rubeš. She receives an invitation from Jane Hillary Leslie Caron, the headmistress of Brookings School for Girls in Italy, to join her school. Heidi has the means to go, having just inherited from Klara's grandmother. Grandfather urges her to seize the opportunity so she can take care of herself when he is gone. Although hesitant at first, the news that her sweetheart Peter Charlie Sheen has joined the army prompts her to accept.
At the school, Heidi struggles to adapt to the more modern life and finds herself clashing with Ursula Joanna Clarke, a classmate with a refined air. Heidi’s only true friend is Ilsa Nicola Stapleton, who often stands up for her against Ursula and her snobby circle, though she finds support in the kindly guidance of Jane Hillary Leslie Caron, who, like Heidi, comes from a modest background.
Italian troops arrive at the school, bearing a letter from the Governor, and commandeer the building for use as a military post. All the girls are fetched by their families except for four: Heidi, Ursula, Ilsa, and Gudrun Kathryn Ludlow. Hillary is glad to care for the four, but the unscrupulous owner of the town orphanage, Signor Bonelli Yorgo Voyagis, has the Governor’s backing and takes custody of the girls. The orphanage is run-down and cruel, relying on forced labor, and Hillary’s attempt to secure the girls’ safety is thwarted by Bonelli.
Heidi hopes to escape, but Ursula insists that Madame Hillary will come for them. Hillary’s rescue plan is blocked by Bonelli, and Heidi learns from another orphan, Clarissa Jade Magri, that there is a possible exit through the drains. The four Brookings girls, Clarissa, and another orphan named Giovanni manage to slip away, hitching a ride on a cart toward the countryside.
Bonelli pursues them and catches Giovanni. The group presses on through the forest toward the mountains, aiming to cross into Switzerland, briefly crossing paths with soldiers on horseback. Hillary learns of the sighting and, knowing Heidi would lead the others to her grandfather, decides to travel to the Alps herself. Grandfather learns of the crossing through Peter Charlie Sheen, and asks him to guide the girls safely across.
The girls push through the rugged mountains, and Peter brings them food and hope. He leaves to fetch a sled, but while he is gone, Bonelli closes in. To protect his secrets, Bonelli plans a staged accident to send the girls tumbling from the mountain. Heidi calls for help, and Peter arrives just in time to shield them; Bonelli struggles with Peter and falls to his death. The group continues their climb and finally reaches safety, reuniting with Grandfather and Hillary.
Back together, they share a Christmas dinner, a quiet moment of relief amid the ongoing war. Clarissa is to be adopted by Grandfather, and Bonelli’s orphanage is shut down, though the war continues around them. Peter must leave again to fulfill his duties, but he promises to return to Heidi. The story closes on a note of resilience and hope, with the group looking ahead to a future shaped by courage, loyalty, and the ties that bind them.
