Guided by their inspirational French teacher, a diverse group of adult students escape the backdrop of modern-day West London and find new meaning in their lives by exploring Frenchman Jean Giono’s celebrated short story ‘The Man Who Planted Trees’ about one man’s hope for humanity via his generosity to nature.

Not an easy concept to write a concise logline for.
I also agree with variable, what does 'exploring' Jean Giono's book look like on screen.
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"Inspired by his French teacher, a West London student and his classmate's vow to do one good deed a day in spite of pushback from his father who believes no good deed goes unpunished."

what constitutes 'exploring'? how do they 'find new meaning in their lives'? what specific action takes place?