In which Martin Arrowsmith, steadfast in his scientific toil amid institutional upheaval, pursues groundbreaking research on phage treatment for pneumonia and plague, confronting both the chaos of leadership changes and the eccentricities of his colleagues. With the unexpected arrival of the irrepressible Gustaf Sondelius as his aide, Arrowsmith navigates the rigours of experimental medicine while the Institute around him flounders in a swirl of intrigue and ambition.