In the dark town of Grislygust a competition is under way - The Great Grislygust Grow-off. Marvin plans to grow the tastiest tomatoes in town but his beastly brother Boris an his poisonous sister Prunella make fun of him. 'You're a titchy weed, Marvin, and your tomatoes will be tiny too!' But Marvin is determined to prove everybody wrong and soon his competitors will find themselves in big trouble.