The story begins in The Rocks precinct of modern-day Sydney where four teens (Jemima, Archie, Millie and Oliver) discover the entrance to a sealed and long-abandoned tunnel leading from the cellar of the 'Hero of Waterloo' hotel.
After venturing into the tunnel, they become disorientated and when they emerge at the other end, they find themselves in bushland, where without modern landmarks, they are unable to get their bearings until looking down the escarpment (where Circular Quay should be). They see wattle and daub huts, soldiers in red jackets and men working in chains. They hear orders bellowed from the parade ground and the metal clanging from a blacksmith's hut. They later discover it is 1791.
While still trying to fathom their new reality, they are startled by Nanberry, an Indigenous youth who they discover, after surviving small-pox, lives in Sydney Cove. Acutely aware of the importance of avoiding further detection, they are grateful when Nanberry offers covert assistance. Through him, the four reluctant time-travellers experience their first overnight bush-camp, learn survival skills and experience a world they could never have imagined.
The storyline follows their perilous adventures and their furtive interactions with two young convicts and several Indigenous youth including Nanberry, Boorong and her older brother Ballooderry who help them hide and survive.
The time-travellers experience the living conditions in Sydney Cove and see the plight of the Indigenous people firsthand. They witness an amputation, the flogging of a convict for stealing fishing-tackle from an Indigenous woman Darìnga, and the daring escape of convicts William and Mary Bryant with their infant children and nine other convicts.
While trying to find their way home they meet and engage with diverse inhabitants including a convict's daughter Mary Mullins, 'wild white man' John Wilson, they observe the developing friendship between Lieutenant William Dawes, a group of Indigenous youth and Bennelong and his wife Barangaroo. They hear stories of Nanberry and Boorong's small pox survival and ongoing care by Surgeon John White and the Reverend Johnson. As well as of Balloderry whose beloved canoe is smashed by convict men.
After witnessing clandestine events, the youngsters are chased and become separated but are reunited on Me-Mel Island (Goat Island) in Sydney Harbour with the assistance of Nanberry and Boorong. Whilst on the island they share campfire stories that provide opportunity to the readers to consider ethical issues and to witness how each of the protagonists has grown in resilience and compassion during their time in Tar-Ra.
The four travellers eventually locate the entrance to a cave which they believe hides the tunnel that will lead them back to modern-day Sydney. As the risk of being discovered and captured is heightened they enter the cave in a panic and are enveloped in darkness.
The ending leaves unanswered questions so the reader can draw their own conclusions. However, an epilogue indicates that they did make it back to the 21st Century.