The one where the Daleks invade the past from the future...
The Daleks are on the cusp of exterminating their arch enemy the Doctor once and for all, gunsticks poised, and at the very last moment, the Controller barges in, shouting: "Stop! You mustn't kill him!" It's one of those age-old devices that writers use to ramp up false peril, but diffuse it with the laziest of get-outs. I hate it when this happens. Also, where's the sting reprise gone?
There's a lot of info-dumping in this fourth episode, but it finally helps to colour in the sketchy grasp we've had so far as to what's going on. We've known that the freedom fighters have been trying to go back in time to kill Styles, but not why. Here, we learn that the Daleks have discovered time travel technology (which contradicts The Chase and The Daleks' Master Plan, but whatevs) and invaded Earth a second time, thus changing the pattern of history (the first time they tried they were defeated by the Doctor in The Dalek Invasion of Earth).