Doctor Who Day of the Daleks by Terrance Dicks

In this narrative, a group of freedom fighters from the twenty-second century travel back in time to prevent a global catastrophe that allowed the Daleks to conquer Earth. They mistakenly target a diplomat named Sir Reginald Styles, believing his actions at a peace conference triggered the future war. The Doctor and Jo Grant become entangled in the conflict, eventually discovering that the resistance's own attempted assassination was the true cause of the timeline's collapse. By intervening, the Doctor prevents the murder and allows a brave soldier named Shura to sacrifice himself to destroy the Dalek invasion force. Ultimately, the original timeline is restored, ensuring the human race avoids centuries of brutal enslavement by their alien enemies.