With financial support from her uncle she studied at the ETH Zurich, where she passed the examination as instructor for natural science in 1893.

Natural science was part of the philosophy department of the University of Zurich and while preparing for her doctorate she met and formed a lifelong friendship with Ricarda Huch, with whom she continued to exchange letters.

[1]: 1032 In 1898 she was appointed as assistant lecturer at the Bavarian Biological Experimental Institute (Bayerische Biologische Versuchsanstalt), at the School of Veterinary Medicine in Munich, Bavaria.

Plehn's research agenda was determined by the needs of breeders supplying stock to lakes and streams in and around Munich and upper Bavaria.

Her research was pioneering and often conducted under time pressure as breeders faced considerable financial losses.

Between 1903 and 1904 she identified the agent causing red disease in carp species, the Bacterium cyprinicida, and demonstrated more hygienic conditions reducing the problem.

In 1909 she was promoted to the position of Konservatorin (curator) at the Starnberg research station and her annual salary was raised to 3,000 Mark.

She spent much of her working life at Starnberg and took her holidays outside the summer season as she and colleagues had to ensure that the research station was always staffed.

She examined in great detail the bacteria causing epidemics of furunculoses (abscesses) and fluorescence (open sores).

[2]: 118 In Germany it was not possible for women to qualify as lecturers, thus Plehn was awarded the title royal professor in 1914 by King Ludwig III of Bavaria for her great contribution to fish pathology.

[1]: 1032  In 1920 Plehn published reports on two skin and gill parasites, the Ichthyochytrium vulgare and the Mucophilus cyprini (later identified as a rickettsia[citation needed]).

Plehn focused on the major brood of fish in Bavaria, the Salmonidae and Cyprinidae (salmon/trout and carp) and included illustrations and practical guidance based on 20 years of her work.

[2]: 118 Although she was a professor, her teaching activities were limited to courses at the Fisheries School at Starnberg, which was attached to the research station.

She continued to be a presence at the Starnberg research station, giving lectures and putting her knowledge at the disposal of colleagues.

Although she was an outspoken critic of Adolf Hitler, the Nazi authorities did not force her to stop her teaching and research work.

[2]: 119 But she was only given a leading position when male researchers were drafted for military service during World War II.