In the second Harry Potter movie - The Chamber of Secrets, Hermione Granger suggests the use of the Polyjuice Potion to investigate if Draco Malfoy is the heir of Slytherin. She does so right after their class is told that Salazar Slytherin had built a hidden chamber in Hogwarts Castle, of which the other three school founders were unaware. According to legend, before departing, Slytherin had sealed the Chamber of Secrets so that no one would be able to open it until his true descendant arrived at the school. Slytherin's heir would be able to "unleash the horror within" to rid the school of children with Muggle parentage. The Hogwarts staff is worried sick, the students are scared, and the plot of the second Harry Potter movie thickens.
Ron assumes Lucius Malfoy's son, Draco is the heir simply because his reputation precedes him. Hermione comes up with a time-consuming, and flagrant rule-breaking idea to trick Draco into telling. Her plan isn't all that simple, but because she is the brightest witch, and has been paying attention in Snape's Potions lessons, she knows all they need is some Polyjuice Potion. Her plan is to get inside the Slytherin common room and interrogate Draco without him realizing who they are. The Polyjuice Potion will transform the three Gryffindors into Slytherins, and Draco thus should likely tell them what he's up to. While Ron and Harry have their concerns about the potion, Hermione address all of them, one by one.
Hermione Brews The Polyjuice Potion
The Trio is easily able to access "Moste Potente Potions" from the library in Harry Potter and the Chamber of Secrets. In the book, however, Hermione says the said book is bound to be in the Restricted Section. She obtains a permission note to check "Moste Potente Potions" out by tricking Gilderoy Lockhart. Hermione tells the professor she needs the book for background reading and to understand what he says in "Gadding with Ghouls" about slow-acting venoms. Lockhart signs a permission note and the trio head to the library. After thoroughly examining Lockhart's note, Madam Pince hands Hermione the tome which she then slides into her bag.
"Properly brewed, the Polyjuice Potion allows the drinker to transform himself temporarily into the physical form of another," reads out Hermione in the book-to-screen adaptation of Harry Potter and the Chamber of Secrets. Ron is immediately sold on the idea and knows Draco will reveal anything so long as he thinks they are his friends, Crabbe and Goyle. While it sounds like the Trio is about to embark on an adventure, the truth is, brewing the Polyjuice Potion is tricky and when Hermione says she's never seen a more complicated potion before, she pretty much means it. Draco has allegedly been attacking more and more Muggle-borns, and Dumbledore confirms to Minerva McGonagall that the Chamber of Secrets has been reopened. Ron, meanwhile, comes up with another theory about the reopening of the Chamber of Secrets. "Lucius Malfoy must have opened it when he was at school here. Now he's taught Draco how to do it," Ron concludes at the second-floor girls' lavatory. Unsure, Hermione reminds him they will still have to wait for the Polyjuice Potion to confirm Draco's involvement.
The brains behind the plan, Hermione is the one to scan through the Polyjuice Potion recipe in Harry Potter and the Chamber of Secrets book. While she says Lacewing flies, Leeches, Fluxweed, and Knotgrass are easy to obtain from the student store cupboard, the powdered horn of a Bicorn, the shredded skin of a Boomslang, and a bit of who they change into are the tricky ingredients. Knowing the extent of the threat that the Muggle-borns at Hogwarts face, she persuades Harry and Ron to break the rules. After informing her friends that Fluxweed has got to be picked at the full moon and Lacewings have got to be stewed for twenty-one days, Hermione passionately declares that they go full steam ahead.
Hermione Turns Into A Cat
Hermione brews the Polyjuice Potion in an old cauldron in Moaning Myrtle's bathroom, and it's only half-finished at the beginning of Chapter 11 of Harry Potter and the Chamber of Secrets "The Dueling Club." The trio still needs Bicorn horn and Boomslang skin, and the only place they are going to acquire it from is Snape's private stores. After they manage to obtain the new ingredients from Snape, Hermione throws them into the cauldron and informs everyone that the Polyjuice Potion will be ready in two weeks. They stay at school for Christmas, and because Draco does too, they become excited to extract a confession out of him.
"We still need a bit of the people you're changing into," says Hermione pointing out that they need either the toenail clippings, dandruff, or hair of the person (Crabbe and Goyle, in this case) they are assuming the physical appearance of. Moreover, the Polyjuice Potion lasts anything between ten minutes to twelve hours. This of course is subject to how well Hermione brews it. Harry and Ron obtain Crabbe and Goyle's hair by drugging them with two plump chocolate cakes filled with Sleeping Draught. As they keel over, Harry and Ron hide them in the broom closet and pull out several of their strands. They steal their shoes and rush to Moaning Myrtle's bathroom.
Hermione splits the Polyjuice Potion into three, and she drops Millicent Bulstrode's hair into her glass. Harry drops Goyle's hair and Ron Crabbe's. The trio takes separate stalls to consume the potion and Chapter 12 of the second Harry Potter book - The Chamber of Secrets "The Polyjuice Potion" describes Harry's experience as "...writhing as though he'd just swallowed live snakes..." Ron is indistinguishable from Crabbe, and Harry from Goyle. Hermione insists the boys go on without her and doesn't come out at all. Draco confirms he has no idea who is behind the chaos, and that his father refuses to divulge information about the last time the Chamber was opened. Hermione still hides inside her stall even after the duo is back to normal. Her face is covered in black fur, her eyes yellow, and she's transformed into a cat. The Polyjuice Potion isn't supposed to be used for animal transformations, and unfortunately for Hermione, Bulstrode had left cat hair on her robes while strangling her at the Dueling Club. As Hermione finally leaves the bathroom to go to the hospital wing, Moaning Myrtle's ghost mocks her, saying, "Wait till everyone finds out you’ve got a tail!"