"It's so much responsibility," she wails, as the high-speed ship slams into the waves. "I don't want to have to do this."
Even though the film follows the #FridaysForFuture mass demonstrations that Thunberg launched through 2019, the era feels like a lifetime ago given the current Covid-19 pandemic and restrictions on big assemblies.
Thunberg, now 17, urged the world to not forget the climate crisis and said the environmental campaign continues "in the way that is the most safe and that doesn't put anyone at risk, in line with Covid-19 restrictions, of course".
In Stockholm on Friday morning, for example, she was out there striking before school, wearing a face mask and socially distanced from others. Thunberg returned to school last month after taking a year off for her activism.
Grossman had remarkably close access to Thunberg and her family as she was becoming an international media phenomenon, and the end result is a film that provides a much fuller, emotional portrait of an ordinary yet at the same time extraordinary teen.
View the trailer for I am Greta here:
Filming behind the scenes during the Atlantic crossing and long train trips to European capitals, Grossman shows a Thunberg who cries, struggles translating a phrase into French and gets frustrated with her father, but who then holds her own in the halls of power.
The film debunks some of the criticisms of Thunberg, showing her writing her own speeches and making clear she was the driving force in the campaign, not her parents or other environmental interests. At the same time though, it makes viscerally real the pressures that were allowed to accumulate on her as the movement grew.
Thunberg said she appreciated that Grossman didn't further what she said was the stereotype of her as "the angry, naive child who sits in the United Nations General Assembly screaming at world leaders".
"That's not the person I am," Thunberg said. "He definitely made me seem more like a shy, nerdy person, which is the person that I am."
Grossman for his part, said Thunberg had only a few requests when he showed her the finished product, asking only to add more — which he couldn't do for space reasons — but not asking to cut anything.
Thunberg said she didn't really notice Grossman's presence, and in fact sometimes wondered what he was up to since he was doing everything himself.
"It was not very professional," she said of his low-frills film-making project. Speaking directly to Grossman, she said: "Sometimes I actually doubted, since you said this might be something big. And I said 'Why don't they send like a sound guy or anything, or why don't they have more professionals?' So I think maybe on some level I doubted the seriousness of the project."
But now it's out, Thunberg said she was pleased with the end result.
And then she excused herself to go back to class.