The Dehn Invariant
It was #3 on Hilbert's list of the most important problems in mathematics - until his student solved it.
Featuring Daniel Litt, currently at the Institute for Advanced Study - https://twitter.com/littmath
David Hilbert's problems: https://en.wikipedia.org/wiki/Hilbert%27s_problems
Max Dehn's famed proof: https://archive.org/details/mathematischean33behngoog/page/n488