In the novel The Secret Adversary, the Moat House is a house in Ebury, Yorkshire. The building is in a state of disrepair. A Mrs Sweeney holds the key to the house.
Tommy and Julius Hersheimmer discover a telegram in Tuppence's room, telling her to go to the Moat House. The telegram is signed "Tommy", but Tommy says that he did not send it. Tommy and Julius then go to Yorkshire in search of Tuppence.
The Moat House is described as being a "big house near the sea". It has been empty for years, and is in a state of disrepair. The rusty gate swings from its hinges, the overgrown drive is covered with leaves, and the steps up to the front door are covered with moss. The windows are all shuttered, and the bell handle is rusty.
Tommy and Julius get the key to the Moat House from Mrs Sweeney, who expresses doubt that it would suit them, as the ceilings are leaking, and it would need a lot of money spent on it.
Inside the house, Tommy and Julius find a thick layer of dust everywhere, with no sign that Tuppence or anyone had been there recently. However, they do find a brooch that belongs to Tuppence.