Soon after the Nazi occupation of Poland, he was active in sabotaging the railroad tracks near Bochnia, and then moved with his mother Aniela to Łopuszna for their safety.

From there, they went to Nowy Targ, where Popek obtained a position as a clerk at the office of Nazi Kriminalpolizei near the town centre, and began collecting classified information for the Polish underground.

[1][5] However, his underground work ended in late January 1942 as a result of the infiltration of their group by the Gestapo agent and former acquaintance, Hainz Wegner-Romanowski.

[6] Popek was arrested and interrogated, tortured and taken to an execution centre at the Palace Hotel in Zakopane (dubbed the "Death's Head Resort" by the locals).

The body of Tadeusz Popek was exhumed by the Polish communist authorities in 1946 as evidence during the court case of Goralenvolk leaders, and re-buried in Zakopane, at the Nowotarska Street cemetery.