The Grémillet sisters are heading to the mountains for a long-awaited reunion with their dad. But as always, things don't go as planned... With their astronomer father preoccupied with a soon-to-arrive comet, Sarah, Cassiopeia, and Lucille set out on their own to explore the region and it isn't long before they stumble onto a new (four-legged) mystery!

Being sisters is never easy. But when you're as different as Sarah, Cassiopeia, and Lucille, it's even harder! The first is haunted by recurring dreams, the second lives with her head in the clouds, and the last spends most of her time with her cat. Then one day they discover a mysterious photo of their mother pregnant. Where was it taken, and who is the baby? And most importantly, why was this photo hidden away in the depths of the attic? To find out, they'll have to venture into the tangled forest of the Grémillet family secrets!

Summer vacation has finally arrived! And like every year, Sarah, Cassiopeia, and Lucille are off to the peaceful little village where their grandmother lives. There they'll find their old friends, the sleepy countryside, and... a ghost! Indeed, a mysterious presence haunts the ruins by the lake. Where does it come from? And why does it ring the bell in the old church? Finding out will be the new mission for the Three Sisters' Club! But to unravel this mystery, they will have to travel back in time, meet fair damsels and knights, and confront tortured love stories past and present...