He studied at secondary school where he was taught in Russian, German, and Polish.

Jointly with Kurt Hensel he edited Kronecker's collected works.

It is a theorem concerning the boundary behaviour of functions meromorphic in the unit disk.

One scholar writes that in Plessner's case "the anti-Semitic prejudice was mixed with and partly hidden by concern for their lack of a German citizenship".

Plessner is widely viewed as a founder of the Moscow school of functional analysis.