The Sixth Doctor and Peri awaken aboard the starship Vipod Moor, where a dual-personality computer has seized control, attempting to propel the vessel back to the universe's dawn and rewrite history. Amidst chaos, they encounter comedy policemen, an art thief, and a captain eager to spread his diseases, while a vicious Maston creature stalks the crew. As the Doctor races to prevent catastrophic temporal disruption, a Gallifreyan council member intervenes, revealing the web of time's intricate safeguards.