Deep among the Swedish pines lies the fantastical world of the trolls. More often than not, these kindly creatures keep themselves to themselves, but this summer is different.

Ten-year-old Poppy's school holidays are transformed when she befriends the young troll Baggledorf at the forest lake near her Grandpa's cottage. Baggledorf is a chunky, hairy, but cuddly individual with a few vile habits, and many exciting hobbies. The fun-loving troll introduces Poppy and her dog, Fred, to quarry cloud racing, deep-forest grass boarding, and pine-tree sliding.

In summer rains and summer sun, from above and below the dense forest canopy, Poppy falls in love with Baggledorf's world. But amidst the wonder is an undercurrent of mischief, as the BEB (Bog-end Belchers) plot Baggledorf's downfall. The BEB are a bunch of notorious trolls, who burp, fart, drop litter, and are fanatical about the prestigious troll sport of Bog Wrestling.

This year, Baggledorf has the honour of organising the yearly Bog Wrestling event at the Troll Summer Festival. Unsurprisingly, this prestigious appointment has invited the envy and anger of the Bog-end Belchers. The trouble begins when the BEB plot Baggledorf's disgrace. The plot culminates at the Troll Summer Festival, when mayhem breaks out under the frosty glare of the famous Bog Wrestling judge, Mr. Tuganpullem. As the web of forest intrigue unfolds, Poppy is left to save Baggledorf's good name.