First three people (Mrs. Sharpe, Ms. Hussey, and Mr. Watch) receive mysterious letters and then the painting A Lady Writing gets stolen. Calder and Petra (who are in Ms. Hussey's class) go searching for the painting because they suspect Ms. Hussey had to do with it. They go to Mrs. Sharpe's house to get advice. Calder and Petra are both fantasized by pentominoes and a book called Lo!. Then they search the University of Chicago campus. Calder gets locked in the University basement. Later the two search delia dell hall where the find the painting. The thief chases them and Calder gets caught and Petra gets away. Calder escapes and they all live happily ever after.
Friendship: in the story Petra and Calder are great friends and care for one another
Calder is a main character who is fascinated with puzzle pieces called pentominoes. He is very intelligent and is willing to do whatever it takes to find A Lady Writing.
Petra is a main character who is very smart and likes a book called Lo!. She is good friends with calder and together they will find A Lady Writing.
Mrs. Sharpe is a major character who is a widow of a vermeer scholar. She befriends the children and helps them solve the mystery.