Polyjuice Potion transforms one's appearance into that of someone else for a limited time period.
Hermione Granger thinks of using this potion to spy on Draco Malfoy to try and find out if he is the heir of Slytherin after she hears Severus Snape talk about it in potions class. Hermione plans to make the potion by looking up the recipe in a book called Moste Potente Potions which is kept in the restricted section of the Hogwarts library. Hermione asks Professor Lockhart to sign a permission form so she can check out the book from the library. The trio then secretly brews the potion in Moaning Mrytle's bathroom. Harry Potter uses the potion to look like Goyle, and Ron Weasley uses the potion to impersonate Crabbe. Hermione plans to use the potion to look like Millicent Bulstrode, but ends up getting a cat hair instead. Hermione ends up with a cat face and in the hospital wing. The potion is not supposed to be used for animal transformations.
Barty Crouch Jr. takes Polyjuice Potion throughout the whole school year in order to impersonate Alastor (May-Eye) Moody. This is part of a plot to create a new body for Lord Voldemort with the use of Harry Potter's blood.
Polyjuice Potion is featured in the ministry of magic pamphlets. Wizards are advised to agree on security questions to try to detect Death Eaters that are using Polyjuice Potion to masquerade as others. Polyjuice Potion is also one of the potions Professor Horace Slughorn quizzes the students on in his first potions lesson. Hermione Granger correctly identifies the potion, and Harry Potter reminisces about how she brewed it in their second year.
Polyjuice Potion is used to create seven potters to confuse the Death Eaters while trying to move the real Harry Potter to a safe house. The trio also used Polyjuice Potion throughout the book to disguise themselves and break into the ministry of magic and Gringotts.
Making the Potion
Polyjuice potion is made with Lacewing flies, leeches, fluxweed, knotgrass, powdered horn of a bicorn, shredded skin of a boomslang, and a bit of whoever one wants to change into. The fluxweed needs to be picked at the full moon and the lacewings have got to be stewed for twenty-one days so the potion takes about a month to prepare.
The Polyjuice Potion is first described as looking like "thick, dark mud, bubbling sluggishly". When Harry is officially taught about the potion by Horace Slughorn and he again describes it as a "slow-bubbling, mudlike substance".  The potion changes color once the bit of the person one wishes to change into is added. When Harry's hair was added to the potion it began to froth and smoke, then, all at once, turn a clear, bright gold. Hermione Granger notes that Harry looks "much tastier than Crabbe and Goyle".  Goyle's potion turned the khaki color of a booger and Crabbe's potion was a dark, murky brown. The cat hair Hermione believes to be a hair from Millicent Bulstrode turns the potion a sick sort of yellow.