Upon release, on 12 February 1935, he returned to his family in Hamburg. On 8 March 1935 he was placed in preventive detention, indicted as the "intellectual author" of a murder in Bonn. Despite the fact that the case was closed, Bästlein was sent to the concentration camp in Esterwegen and in 1936, to Sachsenhausen, where he met Robert Abshagen, Franz Jacob, Julius Leber, Harry Naujoks, Wilhelm Guddorf and Martin Weise. While at Sachsenhausen, Bästlein helped write the "Sachsenhausen Song", which was at the demand of the SS guards, who would use music to torment and mock the prisoners, making them sing while involved in hard labor or when they were exhausted. The prisoners, however, used the singing as an opportunity to uplift their spirits and encourage prisoner unity and an anti-fascist spirit. In April 1939, he was sent to the Cologne prison, Klingelpütz, where he stayed in police custody till 6 April 1940. Returning to his family, then living at Goldbekufer 19 in Hamburg, he worked as a car washer and driver, then later in Altona, at Riepe-Werken, making ballpoint pens. Bästlein began getting together with friends from Sachsenhausen, such as Abshagen, Jacob and Oskar Reincke, who all wanted to get back to work in the German Resistance. In 1941, they built the Bästlein-Jacob-Abshagen Group, with the objective of educating workers and organizing acts of sabotage. They were active in the Hamburg shipyards, developing over 30 factory cells and supporting prisoners of war and forced laborers. In time, they built a network of contacts in northern Germany, in Flensburg, Kiel, Lübeck, Rostock and Bremen and even with groups outside of Germany. These connections were each overseen by a single leader to lessen the chances of the whole network being exposed to the Nazi authorities. In the middle of 1942, there was a major leaflet campaign directed at construction workers, primarily in Hamburg, who were forced to work with the Organisation Todt in Norway and the Soviet Union. The leaflets linked the general socio-political demands for wages and severance pay with the call to commit acts of sabotage. It closed with the slogan, "Hitler's defeat is not our defeat, but our victory!" In mid-May 1942, four people entered Germany illegally by parachute, jumping from Soviet planes over East Prussia.