The Girl in the Secret Room (密室の少女 Misshitsu no Shōjo) is the fourth episode of Phi Brain: Kami no Puzzle Season One and the fourth episode of the anime series overall.
Kaito is given a Sage Puzzle to solve on the weekend, where he is to be partnered with Ana Gram, an art enthusiast who holds the title of 'Da Vinci'. They visit the mansion of a late artist, with its butler revealed to be a Giver from the POG. Kaito and Ana are trapped in a room which will fill with gas if they don't solve the puzzle. Presented with a jigsaw containing various famous paintings, Kaito and Ana work together to solve the puzzle, the treasure of which is a series of paintings the painter dedicated to his wife. Back at school, Kaito and Nonoha are surprised to find that Ana is a boy.
Another puzzle is set for Kaito to solve along with a mysterious partner titled "Da Vinci". He goes to the second art room with Nonoha to see his partner, and finds a girl named Ana Gram talking to bats. She soon undresses causing Nonoha to immediately grab Kaito's head to cover his eyes and slam him onto the ground outside the room. A girl from the Puzzle Club comes with a jigsaw puzzle, but she trips, and the puzzle scatters onto the ground. Even though the pieces are scrambled, Ana instantly recognizes the picture on the puzzle as one of Leonardo da Vinci's and that a piece is missing. She reveals that, like Cubic, she is uninterested in puzzles. Nevertheless, she goes with Kaito because the location of the puzzle is at the mansion of a recently deceased master painter. The painter's wife and butler were left with the puzzle assigned to Kaito and Anna. They lead the solvers to a room with many paintings of various famous artists on the walls. The POG Giver turns out to be the butler who explains that the puzzle is to be solved before they drown in a thick air that will be filling the room. Meanwhile, Nonoha and Gammon try to get into the mansion since Nonoha is worried about Kaito and Gammon wants the prize. Back at the puzzle, while Ana admires the artwork, Kaito realizes that the paintings don't all have the same edge pattern on the frame. Kaito and Ana align the paintings according to corresponding edges, and by using the initials of the artists of the paintings, they come up with the letters M-D-C-L-V. Unfortunately they discover that the panel only takes in numbers. Kaito gets frustrated and looks towards the clock which he notices uses Roman numerals. Applying this to the letters they just found gives them the number 1655. The code works, and the puzzle is solved. When the painter's wife is told of the combination number, she remembers how the master painter and her met at the address 1655 in Paris. The butler reveals that he was instructed by his master to design the puzzle, and that the gas was actually just dry ice, referencing to how the master painter loved playing tricks. The prize of the puzzle is revealed to be many paintings of his wife that the painter made throughout his life.