Once upon a time... there were Three Doctors. Two Osgoods. One peace treaty.
So begins the Doctor and Clara's latest adventure, which details the consequences of allowing 20 million Zygons to call Earth home. A rogue faction break the peace treaty, and we're off!
Having detailed "Operation Double" (aka the Zygon peace treaty) via video message, the Osgood twins reveal the Osgood box. It is the last resort or "nightmare scenario" in case the peace treaty fails.
Osgood's message reaches the Doctor on the TARDIS. He immediately reaches out to the Zygon high command and learns that a rogue faction is behind these turn of events. Kate at UNIT informs the Doctor that Osgood has been kidnapped and along with her the location of every Zygon on Earth.
The Zygon splinter cell forces Osgood to relay a message that UNIT troops will be destroyed around the world. "The war is about to begin" she states.
Meanwhile, Clara is oblivious to what's going on and realizes she has 127 missed calls from "Doctor Disco." She comes across a small boy who claims his parents are missing. Inside their apartment, Clara encounters the couple who are acting strangely. The boy is taken away screaming.
At the former Zygon high command secret base, the Doctor and UNIT discover the control center for all Zygons on Earth. The Doctor informs everyone it has been compromised. A video transmission confirms the rogue faction is now in control.
Though the Doctor wishes to negotiate, Kate suggests bombs pointing out the Zygons broke the treaty. He manages to talk her down. Kate heads to New Mexico for more intel, while the Doctor attempts to rescue Osgood. He requests his Presidential aircraft.
Clara returns to her apartment for some things, but the elevator drops her and Jac in a tunnel system. The duo goes in search of reinforcements.
When the Doctor arrives at the UNIT command base in Turmezistan, he teams up with Colonel Walsh to locate Osgood. At a nearby Zygon training camp, UNIT soldiers fall victim to Zygons posing as their relatives and are killed. The Doctor finds Osgood locked in a cellar, they make their way to his plane.
"Which one are you? Human or Zygon?" the Doctor asks Osgood. She refuses to answer, informing him that she is a hybrid of the two.
Kate arrives in Truth or Consequences, New Mexico and learns from Sherrif Norlander that Zygons took over her town. Later on, Norlander reveals herself to be a Zygon and moves on Kate.
Underneath London, Clara, Jac and UNIT soldiers come across the Zygon pods. Clara discovers a duplicate of herself inside one of the pods. Jac quickly realizes the original is inside the pod, and Bonnie has them all killed.
A flashback reveals Clara was duplicated while in her neighbors apartment and Bonnie took her place. The Zygon tracks the Doctor's plane and contacts him while taking aim with a rocket launcher.
"I'm sorry, Clara is dead. Kate Stewart is dead. The UNIT troops are all dead. Truth or consequences" Bonnie tells the Doctor as she fires.