Harper uses the backpacker murders--a spate of serial killings committed by Ivan Milat in New South Wales between 1989 and 1993--to inspire her second novel, Force of Nature. Featuring solitary Federal Agent Aaron Falk, Harper sets her story in the Giralang Ranges, an isolated, picaresque ocean of mountains a couple of hours drive from Melbourne. From a series of broken friendships to a brother and sister on edge, five colleagues take time off their dull jobs at a boutique accountancy firm to go on a team-building exercise. Alice Russell, the most experienced of the five, has got herself lost. She was last seen when the group were dropped off at the start of the Mirror Falls trail. Four of the women found the meeting point but have various cuts and bruises. One of the girls, Bethany, is getting treatment at the local hospital, suffering from a snake bite.

More unsettling is the issue of the Martin Kovac case from twenty years ago. A transient laborer, Kovac killed several women while they were hiking in the Ranges. Kovac's name was first mentioned in connection with the murders, his presence in the area casting a long shadow. Aaron and his new partner, Carmen, race to get to the camp before nightfall. BaileyTennants' chief executive, Daniel Bailey, can shed little light on what happened. The five women are all very different personalities. According to the lead investigator, Senior Sergeant King, Alice has been on her own now for thirty-odd hours and might be suffering from hyperthermia. King tells Aaron and Carmen that the priority is finding her and bringing her out safely. While the searchers are swallowed up by the woods, their orange suits lost to sight, the media hovers at a safe distance, hungry for any information on the missing hiker.

The story is deftly plotted with just a hint of Deliverance. While the police investigate a potential link between the notorious Kovac and the rundown cabin where Alice was last seen, a compelling subplot reveals that BaileyTennants is under investigation by the Australian Federal Police. Aaron wonders whether Daniel and his sister, Jill, knew their company--a respectable firm family-owned for generations--was being investigated, and that Alice, the Fed's contact, had called Aaron on the night she disappeared. Aaron wonders what was going through Alice's mind as she entered the bushland and walked toward something that eventually stopped her from leaving.

Though Aaron's effort to come to terms with the memory of his father is central to the story, the chief focus is on the trials of the five women trapped in a shifting, altering landscape. Twins Breanna and Bethany McKenzie, Lauren Shaw, and Jill Bailey don't recall much about Alice's last moments, beyond that in the morning when they woke up, Alice and her cell phone were gone. Jill and Alice barely spoke, the two initially clashing after Jill refused to give Alice permission to leave the trip early. There was remorse about the morning as Bree (and Bethany) tell of Alice's outburst. Amid the mud and the cold and the squeak of invisible birds, Bree's thoughts plunge "down a black hole...landing with a thump" before the specter of Martin Kovac and his final victim: Sarah Sonderberg, 18 years old and never found.

The days pass. Alice remains missing. Falk grew up in a place like Giralang, an isolated terrain where the thick trees are reluctant to let anything escape. Unconvinced that Bree, Bethany, and Lauren are telling the complete truth, Aaron harbors a nagging feeling that the women were too quick to abandon Alice once they discovered her missing: "he could almost hear an insistent whisper in his own mind." Moving back and forth between Aaron's search for Alice and the plight of the women, Harper highlights the festering bitterness and long-held resentments within the group. Stranded in a landscape of alarming beauty that is indifferent to their plight, the women cannot think straight. With the weather turning, the need to for them to find shelter is paramount. Trapped in an impossible conundrum, with no choice but to keep going, every decision the women make becomes fraught with obstacles.

Not the most dynamic of characters, Aaron Falk is always endearing, an introspective, solitary loner who takes us on rocket-fueled ride to the finish line as he attempts to keep tabs on all the moving parts and suspects in the search for Alice. In what is essentially a man-versus-nature detective story, Force of Nature reinvents what can happen when urban women are plunged into a harsh and volatile world.