With its grassy hills and breathtaking city views, London's Hampstead Heath is the perfect place to spend an afternoon with friends and loved ones--and on an unseasonably warm Valentine's Day, the lawns are especially full. So when an aggressive lovers' quarrel breaks out, there's an audience of park goers nearby to hear the shouts traded back and forth, and to watch as the violence escalates suddenly to murder, then suicide.

For the five strangers who observed the gruesome act, the memory of the gore is unshakable. But one of them--disgraced journalist Jen Hunter--is compelled to question the truth of what she thought she saw. Are the facts of the case plain as day, or were they obscured, in the moment, by the glaring sunlight?

As she mounts an obsessive investigation for a seemingly-impossible alternative, the lives of the other witnesses begin to unravel, each in its own particular way. Soon one thing becomes clear: the crime they witnessed was more terrible, more twisted, and more far-reaching than they ever could have imagined.