Sir Reginald Styles (Wilfred Carter), trying to organize a peace conference to prevent World War III, finds a black-clad soldier armed with a futuristic weapon bursting into his study at Auderley House, but the man vanishes before he can fire. UNIT is called in, but Sir Reginald can tell them little. He is busy organizing a trip to Peking. Later, the gunman re-appears in a time-vortex and is attacked by alien thugs called Ogrons. UNIT finds the soldier injured and takes him to the hospital. The Doctor declares his weapon an ultrasonic disintegrator and finds a crude time-machine in a small box. As he tries to activate it, the injured man vanishes from the ambulance.
Sir Reginald is off to China and the Doctor spends the night at his house, examining the wine-cellar, with Jo accompanying him and UNIT patrolling the grounds. The next morning, three guerillas appear from the time-vortex—Anat (Anna Barry), Boaz (Scott Fredericks), and Shura (Jimmy Winston). At first, they believe he is Sir Reginald and want to kill him. He has re-activated the time-machine and they beg him to turn it off. In the future, the Daleks are alerted. The Doctor convinces them he is not Sir Reginald, but they still don’t trust him, and when UNIT soldiers enter the house, the Guerillas bind and gag the Doctor and Jo in the basement.
In the dystopian 22nd Century, the Daleks order the human Controller (Aubrey Woods) to send Ogron troops to the co-ordinates of the house. The Brigadier (Nicholas Courtney) tells the Doctor on the phone that the conference is on and the World Leaders will be arriving at the house. The Doctor, as his captors are listening, asks the Brigadier to “tell it to the Marines”, making him suspicious. Jo frees herself and threatens to destroy the time-machine, but it activates, and she vanishes into the future, where the Controller charms her and puts her at ease. She tells him everything. Daleks and Ogrons attack the house. Most of the time-travel is happening in a tunnel that exists in both times. The Doctor enters, travels to the future, and discovers the Daleks, human slave-laborers, and the Earth largely destroyed. There was a World War and the Daleks moved in to take over.
The Doctor is arrested by the Controller, but the rebels free him. They tell him it all began at Auderly House. During the peace conference, it was blown up and killed most of the World’s leaders, triggering the war. The rebels believe Sir Reginald did it, the Doctor does not; he realizes this is a predestination paradox. Shura, one of the rebels, thought killed, has survived and will set off the bomb to kill Sir Reginald. Thus, they will start the very war they went back to prevent. The Doctor and Jo make their way back. The conference is attacked by Daleks and Ogrons, UNIT evacuates the delegates, the Doctor creeps into the basement to convince Shura not to set off the bomb. Shura understands, realizes that the Daleks are coming in, orders the Doctor and Jo out of the house, and sets off the bomb, killing himself and all the Daleks.
In the 2011 DVD release, CGI was used to make it look like an army of Daleks was attacking the house instead of the three the original production could afford. Marvel Comic artist John Byrne has admitted that the plot of X-Men: Days of Future Past was unconsciously lifted from Day of the Daleks. But of course, Harlan Ellison famously sued the Terminator movie because its plot was the same as a story he wrote for Outer Limits, which is remarkably like this one, which pre-dates them all. It’s kind of an obvious story when you think about it. Nevertheless, under Terrance Dicks, one of the most science-fictiony of the Doctor Who writers, this is a cracking and sophisticated time-paradox story.