[2] He also wrote several works on religion, including a treatise on the stigmatic Therese Neumann shortly after his ordination, and in 1930 a pamphlet on the International Eucharistic Congress in Carthage, which he attended.

[3] After the occupation of the Czech lands by Nazi Germany in 1939, Plojhar was arrested by the Germans; from September 1939 until 1945, he was imprisoned in the concentration camps Buchenwald and Dachau.

[2] In the post-war period, Plojhar emerged as a leader of the left-wing of the ČSL, supporting cooperation with the Communist Party of Czechoslovakia (KSČ).

After being expelled from the ČSL by the right-wing majority, he and other members of the left-wing faction took control of the party secretariat and reorganized it, openly aligning it with the KSČ.

After the Warsaw Pact invasion of Czechoslovakia and the following period of normalization, he again became president of the ČSL on an honorary basis, and also came to serve as a deputy to the Chamber of the People of the Federal Assembly from 1971 until his death.