I have never heard this term used in Australia. The British Italkians may be able to explain things better. There is a story called The Mouse Plot by Roald Dahl. It is based on the time when he was seven. Roald attended Llandaff Cathedral School in Cardiff. He and his friends had a grudge against the local sweet-shop owner, a bad tempered woman called.Mrs. Pratchett. They played a trick on her by placing a dead mouse in a lolly jar while his friend Thwaites distracted her by buying sweets. They were caned by the headmaster as a punishment, while Mrs. Pratchett watched laughing. Dahl's stories usually had unexpected endings. So you could say a mouse plot is a sort of trick you would play to get revenge on someone you didn't like.

Not a common phrase, i've never heard of it. Since there is a book, the phrase can be referring to the book's "mouse plot." Therefore, "mouse plot" would mean whatever it means in the book.

"The Mouse" is a movie. http://www.imdb.com/title/tt0118056/