Eustacia Rose is a Professor of Botanical Toxicology who lives alone in London with only her extensive collection of poisonous plants for company.

She tends to her garden with meticulous care. Her life is quiet. Her schedule never changes.

Until the day she hears a scream and the temptation to investigate proves irresistible. Through her telescope, Professor Rose is drawn into the life of an extraordinarily beautiful neighbour, Simone, and nicknames the men who visit her after poisonous plants according to the toxic effect they have on Simone. But who are these four men? And why does Eustacia Rose recognise one of them? Just as she preserves her secret garden, she feels inexplicably compelled to protect her neighbour, but Eustacia soon finds herself entangled in a far more complicated web than she could ever have imagined.

When her precious garden is vandalised and someone close to Simone is murdered with a toxin derived from a rare poisonous plant, Eustacia becomes implicated in the crime. After all, no one knows toxic plants like she does...