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Three people got mysterious letters from a random person saying that they should help solve a mystery that is centuries old. He also said that if they call the police that he will come after them.
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Calder and Petra got a weird homework assingment from their teacher Ms. Hussey.
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Ms. Hussey's class (including Calder and Petra) went on a field trip to the Chicago Museum.
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Calder got a letter from his friend Tommy, who moved to New York, saying that his neighbor, Frog got kidnapped.
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Petra called a museum to see where the painting "A Lady Writing" was. Petra then found out that the painting was coming to Chicago.
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On Halloween night Calder met Petra on Petra's front porch and he quickly recognized Petra's costume as the lady in the Vermeer painting A Lady Writing.
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Every time a coincidence or something weird happens Calder and Petra each eat a blue M&M because blue is is mysterious color to them.
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When the painting A Lady Writing arived in Chicago, the museum found out that the painting was missing!!! Petra and Calder then found out about A Lady Writing the next morning after reading the newspaper.
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In the newspaper the day after the painting was stolen, the thieft said that A Lady Writing was safe and he would not hurt the painting. He then said he would give the painting back when the public agreed on which Vermeer paintings were Vermeer's and which paintings were not Vermeer's.
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The day after the painting was stolen, Ms. Hussey came to school and she had a broken arm that was placed in a sling.
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Petra and Calder have tea with a grouchy and grumpy old lady named Mrs. Sharpe. They find out that she owned a book that Petra now owns called Lo!
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Fred left Calder's friend Tommy and Tommy's mom after having them move to New York.
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Petra and Calder raise money to get Tommy home after Fred abndoned Tommy and his mom in New York.
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Mrs. Sharpe asked for police protection because she got scared from the letter she got from the thief.
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After Mrs. Sharpe asked for police protection Ms. Hussey got arrested as a suspect of stealing the painting A Lady Writing. But then she said she got a letter from the thief as well as Mrs. Sharpe and then got realesed from jail.
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Mrs. Sharpe breaks her leg after slipping in her house and has to go to the hospital.
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After Mrs. Sharpe broke her leg, she saw Petra and Calder and asked them to send a letter but then at the post office Petra and Calder decided that they were going to read the letter that was to their teacher Ms. Hussey. Right before Calder and Petra could read it, a man took it and mailed it so they didn't get to see what it said.
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While Petra and Calder are searching Delia Dell Hall they see Petra's dad carrying a package that looked like it could be A Lady Writing then he left with a man that recognized Petra and Calder when they entered Delia Dell Hall.
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At Delia Dell Petra and Calder found A Lady Writing in the staircase under the twelfth stair.
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After the stealer of the painting finds out that Petra and Calder find the painting the theif gets the painting back and soon after Calder finds A Lady Writing in his neighbors treehouse.
