In the final decade of the 19th century, mysterious widow, Hester Appleyard, arrives in Australia, haunted by a secret past. Here, in the new world, she establishes a successful school for young ladies. But six years later, on Valentine's Day, 1900, the enigmatic headmistress allows her pupils to picnic at Hanging Rock. And there, in the strange brooding landscape, something unthinkable happens.

Hester's three star pupils and a governess have disappeared. Led by a dogged sergeant, local men search for the missing. Has something supernatural befallen them? Or has a human outrage occurred? An aristocratic young Englishman, Michael Fitzhubert, was the last person to see the girls, and when the search is abandoned, he is compelled to return to the Rock where he makes a shocking discovery.

The missing Rothschild heiress, Irma Leopold, has been found, remarkably unharmed, but claiming to remember nothing. Meanwhile, rumours spread and the sergeant turns his attention to the college. What secrets lie hidden behind the locked gates? His investigation intensifies Hester's paranoia and she grows certain that the young orphan, Sara, knows something about the disappearance.

Hester struggles to contain mounting hysteria as aftershocks from the disappearance continue to disrupt the old order. A mysterious visitor summons memories of Hester's dark past, whilst Sara seeks escape from the college. Meanwhile, we are taken into the story of the missing girls in the months leading up to the picnic, and the dark stain of the unsolved mystery continues to spread.

French governess, Mademoiselle, is growing increasingly suspicious of Mrs Appleyard's erratic behavior, and fears the worst when she is told that Sara was unexpectedly collected by her guardian, unseen by any other staff. As Mademoiselle searches for answers, the Sergeant travels to Melbourne to seek a last interview with Irma, and a secret vow made by the missing girls is revealed.