The Marxist history of science has played an enormous role in the development of the history of science. Whether through the appreciation of its insights or the construction of a political fortress to prevent infusion, its presence is felt. From 1931 the work of Marxists played an integral part in the international development of the history of science, though rarely have the connections between them or their own biographies been explored. These networks convey a distinct history, alongside political, methodological and personal implications, impressing on us a greater understanding of the possibilities that were present and were lost in the most turbulent of decades. Two of the most notable were Boris Hessen, a founder of Marxist history of science, and J. G. Crowther, one of its most prolific exponents. My examination explores aspects of the dialogue between these controversial figures, starting with brief biographical sketches. Their lives became briefly entwined following the Second International Congress of the History of Science and Technology in 1931, demonstrated with reference to the meeting and the correspondence between them until Hessen's death. In doing so, some new facts and old controversies surface, though most importantly the nature of the correspondence carries implications for the Marxist history of science and for the wider movement of which it is part. The Russian delegation to the congress declared that science was at a crossroads. The history of science was at a similar crossroads in the 1930s.