The TARDIS is drawn off course and sent to Washington DC in the near future by the Time Lords where they want the Doctor to investigate interference in a major historical event to happen within the next 24 hours -- the complete and total destruction of the US capital by an unknown force. With the help of UNIT's scientific advisor, Dr. Black, the Doctor is able to determine the cause of a series of unexplained appearances and disappearances (including that of a cybernetic warrior from the year 2165 named Ray). A time rift, which is a tangle in the delicate web of time, has centered itself on the DC area and, if it is not taken under control quickly, will go critical and destroy the entire city. General Kramer requests the Doctor's aid in averting the disaster.

The Doctor finds himself in a desperate situation. Should he obey the Time Lords and allow the rift to detonate, killing millions of innocent people? Or should he listen to Ace's pleas for him to prevent it? Who is the inside agent leaking information to Captain Walker, the commander of the US Naval Research Station, who seems intent on interfering with every step of UNIT's investigation? What is his motivation? Who is he working for? And what about Ray, the time-stranded cybernetic hunter, who's only joy in life is killing Daleks? How long can she hold out against the self-destructive instincts of her botched cybernetic programming? Then there's Ace, who's own view of the Doctor has suddenly become less clear and more distrustful than before. Will things between her and the Doctor ever be the same again?