Susan, Ian and Barbara are sentenced to be guillotined but an encounter with a dying English prisoner may offer Ian a chance of survival.

Susan and Barbara are rescued by Jules and Jean, two members of the escape chain, while the Doctor gains access to the prison by posing as a revolution official.

The Doctor is forced into a meeting with Robespierre while Barbara's attempts to find a physician for Susan result in them being betrayed.

Ian finds himself held prisoner by the treacherous Leon while the Doctor attempts to engineer Susan and Barbara's escape.

Ian finally delivers his message to James Stirling but in order to secure Susan's release he and Barbara are forced into a dangerous spying mission.