In Paris, inventor Antoine Picard and his sister Francoise makes a momentous discovery; they identify oxygen. They are celebrating their achievement when the Republican guard arrives to arrest them. Francoise escapes with Antoine's notes.
Picard is brought before Robespierre who orders him to work on some plans for an incendiary device. Robespierre believes that Picard's work with combustion would enable them to produce a bomb. Picard refuses to co-operate and is sentenced to die at the guillotine. The Scarlet Pimpernel mounts a spectacular rescue of Picard using a small bomb created by Francoise.