Agatha Raisin longs for a little romance and excitement in the English Cotswolds. Her latest plan to endear herself to bachelor James Lacey, her neighbor, is to take up gardening: She sees the prestigious Carsley Horticultural Contest as the perfect opportunity to impress James.
Unfortunately, Agatha's sincere horticultural efforts are thwarted by a hard freeze that kills all of her seedlings, forcing her to resort to her usual treachery.
When conniving Roy Silver, her former assistant at the PR firm that she sold and retired from, is sent to persuade Agatha to return to London to save the Pure Cosmetics account, he sees the perfect opportunity. Knowing how proud and competitive Agatha can be, he makes her an offer she can't refuse. Roy promises to arrange to fill her garden with healthy, beautiful plants in secret if she will save the account.
As the contest approaches, however, plants are mysteriously being uprooted, poisoned, and burned. When the prime suspect, a beautiful blond newcomer named Mary Fortune, who has her eye on James Lacey, turns up dead, Agatha and James must put aside their differences to solve the puzzle.