Thomas is the 12-year-old son of a millionaire who lives in a big manion surrounded by woods in France. When his mother dies, his father hires a widowed maid to take care of everything while he is away. The woman brings her only son, Charles, to live with them and, hopefully, be company to lonesome Thomas. The rich boy and the poor one become enemies from the very firt encounter. Moved essentially by jealousy and fear, Thomas decides to turn Charles' life into a hell on Earth, especially after their parents eventually fall in love. As the title suggests, he wants to make it clear to the invader who is the lord of the castle.

Thomas, the son of a millionaire lives a fairly isolated existence, in a mansion in rural France. His father hires a widowed woman to take care of things while he is away. The maid's son, Charles moves in as well, and the two parents hope that the two can become friends but they become enemies immediately after meeting each other. Once their parents fall in love, Thomas decides to make Charles, who he views as an "invader", as miserable as possible.