Flashpoint
(aka The Dalek Invasion of Earth - Part Six)
Ian damages the bomb before it is in place and escapes from it before it is dropped into the ground. He then wedges some branches into the shaft after finding a door out of the shaft.
Barbara and Jenny are taken to the Dalek command center. Barbara keep the Daleks distracted while Jenny messes with the controls. The Daleks lock them up once they realize that they have nothing to gain from them.
The Doctor sends Susan and David to destroy some of the Dalek radio towers while he and Tyler head down into the Dalek command center. The Daleks that were in the command center are gone. The Doctor frees Barbara and Jenny, who were left there to die.