Barbara Thorson, portrayed by Madison Wolfe, is an imaginative and fierce young teenager who resides with her supportive older sister, Karen Poots, and her brother. Inspired by her passion for Dungeons & Dragons and the history of former Phillies pitcher Harry Coveleski, Barbara has developed a rich fantasy world of her own. She is convinced that giants from other realms are plotting to attack her hometown, prompting her to spend her days meticulously crafting weapons and traps to safeguard against these threats.
When she encounters Sophia, a newcomer from Leeds, England, Barbara initially seems aloof despite Sophia's attempts to befriend her. A confrontation with a group of bullies led by Taylor interrupts their budding connection, and soon after, Barbara is summoned by the school psychologist, Mrs. Mollé. Annoyed by this interruption, she exits the meeting abruptly, insisting it would only detract from her vital preparations to combat a giant.
As their friendship begins to develop, Barbara shares the mythology of the giants with Sophia, showing her the various traps and baits she has painstakingly created. Convinced that the magical war-hammer Coveleski, which she keeps in her bag, is crucial in her fight against the giants, Barbara also speaks of the eerie Harbingers—ghostly figures that warn her of impending threats.
After a run-in with the principal, Mr. Mollé, resulting in detention for verbally challenging a teacher, Barbara drags Sophia along on her quest for a giant, causing Sophia's confidence in Barbara's claims to waver. When Mrs. Mollé inquires about Barbara's passion for baseball, it triggers painful memories, making Barbara dash home in a daze. Uneasy and confused, she accidentally strikes Sophia, leading to an embarrassing situation that drives Sophia away.
As tensions grow, Sophia finds herself in the clutches of peer pressure from Taylor, who promises her a secret about Barbara if she joins him and his friends in exploring Barbara’s "freaky" activities. This curiosity leads to a harsh confrontation, where Barbara discovers her traps have been dismantled, and she is beaten by Taylor’s gang after revealing Coveleski is merely a small jawbone on a stick.
Recovering in Sophia's home, Barbara becomes distressed at the sight of her surroundings, warning Sophia to retreat before "it" takes notice. This encounter drives a wedge between them, especially after Sophia uncovers a recording that reveals Barbara’s tumultuous past with her mother. The ensuing argument intensifies when Barbara lashes out, convinced her giants are real.
On a quest to confront a looming giant in an abandoned train yard, Barbara uses creative tactics to combat the threat, electrifying it using a makeshift setup before retreating. Following this, her emotions erupt in a prank that humiliates Taylor, prompting concern from Mrs. Mollé and Karen, who search for her.
As a storm brews, Mrs. Mollé implores Barbara to reconnect with her mother, yet Barbara remains oblivious to the impending tragedy. As destruction unfolds around her, the arrival of the giant forces Barbara into battle once more. With Coveleski restored and transformed into a radiant war hammer, she channels her fury and sorrow into an epic clash, uncovering profound truths along the way.
In the aftermath, the titan's haunting words resonate deeply: "To run from death is to reject life." As Barbara returns to the shore, she comes full circle, reuniting with Sophia.
The next day presents an emotional reckoning as Barbara musters the courage to visit her dying mother for a heartfelt farewell. Following the summer, as life carries on in the wake of loss, Barbara rebuilds her connections with Mrs. Mollé and Sophia. On the first night of school, she is visited by the titan once more, a silent guardian watching from the waves, leaving her with a quiet sense of gratitude.
