This walkthrough of the Main Story quest “The Polyjuice Plot” will guide you as you impersonate Headmaster Black using the classic draught of deception, Polyjuice. You’ll get a chance to have some amusing conversations with students as you attempt to discover the password to the Headmaster’s office.
Quest Type: Main Story
Location: Professor Fig’s Classroom, Hogwarts Castle (map)
By now, you should have managed to grab the Floo Flame outside of Professor Fig’s office, so open up your map of Hogwarts and fast travel to his classroom (or the nearest one you’ve got); you can also press to be guided if you’d like to take the long way. Go into his classroom, continue to the back and up the stairs, and enter his office. Walk up to Professor Fig’s desk, stand in the golden circle, and press / to begin a cutscene.
Speak to Madam Kogawa about Scrope
Once the cutscene ends — and you look like Professor Black — you’ll need to go and find Madam Kogawa. Tap to be guided by the compass, or use the minimap’s yellow dots, to make your way to her. On the way, you’ll get to have a number of funny conversations, some optional. First, you’ll be stopped by Professor Sharp — pick whatever dialog choices you like, then continue on your way.
Just before you go outside, you’ll haer Garreth Weasley say he thought he heard the headmaster approaching — if you want, you can turn around and speak with him by pressing /(1). When the conversation ends, continue following the path, which will lead you outside briefly, and then back into Hogwarts. Near the Christmas tree you’ll hear Ominis Gaunt note your presence — once again, you can choose to speak with him if you like (2).
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Your path now leads outside, and on the bridge you’ll reach Madam Kogawa. After a contentious conversation, she’ll direct you to the Great Hall to find Scrope.
Learn the password from Scrope
If you were using the compass’s golden thread, you’ll have to press again. Make your way forward and up the stairs, then through the viaduct courtyard. On the steps that lead to the Great Hall, you’ll find Cressida Blume, who offers the third and final optional conversation you can have (1).
Then go inside, up the stairs, and through the two sets of doors to enter the Great Hall’s foyer, where Professor Weasley will speak with you. When you’re done talking to her, continue into the Great Hall, and make your way to the front of the room, where Scrope is cleaning the podium (2). Approach the house elf and press / to speak with him, allowing you to learn the password you need.
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Once the conversation ends, you can play a final prank before the Polyjuice potion wears off by walking up to the podium and press /. Then, make your way up the stairs in the northwestern corner of the hall (1). At the top of the stairs, head over to the folding screen (2), and once you’re close enough, you can press / to Interact with it.
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After a moment, you’ll be back to your old self, and the quest will be complete! Now you can access the Headmaster’s Office, allowing you to begin the next Keeper’s Trial.
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to be guided if you’d like to take the long way. Go into his classroom, continue to the back and up the stairs, and enter his office. Walk up to Professor Fig’s desk, stand in the golden circle, and press 
/
to begin a cutscene. 
