AKA "Sabotage Agent" A beautiful girl! A dangerous mission! A Brit (Robert Donat) who happens to be fluent in both Rumanian and German -- and a top-notch chemist into the bargain -- is sent behind enemy lines to destroy a poison gas factory. Starts off as a credible spy thriller, with the Nazis really earning audience hatred, then morphs into a James Bond-style fantasia, complete with an enormous secret underground factory and an overdone superweapon. But no hollow volcano. Thoroughly entertaining, but a bit of fluff, even as war propaganda films go.
You can stream The Adventures of Tartu (1943) free and legally on OldReel, via the Internet Archive.
Yes — it is in the public domain and can be watched and shared freely.