Doctor Who and the Day of the Daleks

A shadowy assassin with future weaponry stalks the country home of diplomat Sir Reginald Styles, who's organizing a peace conference meant to head off a looming World War III. When the Doctor, Jo Grant and the Brigadier investigate, they find traces of a crude time machine, and that night, armed strangers step out of thin air, claiming to be resistance fighters from a future Earth conquered by the Daleks, sent back to stop the war by killing Styles. The trouble is that everything they do to prevent that future might be exactly what causes it. Terrance Dicks's novelization of the Third Doctor television serial Day of the Daleks.