In May 1857, Pfeiffer set out to explore Madagascar. Her first stop was Cape Town, South Africa. There, she encountered the French civil engineer and slave trader Joseph-François Lambert. Unbeknownst to Pfeiffer, Lambert had joined Jean Laborde and a few other Europeans in a plot to replace Ranavalona I, the queen of Madagascar, with the more moderate crown prince, Rakoto (the future king Radama II). Pfeiffer unwittingly became part of the conspiracy and was expelled from Madagascar in July 1857 after the queen discovered the attempted coup.