The Enterprise goes on a top-secret mission to take Queen Elaan of Elas to neighboring planet Troyius where she has been promised to their ruler. Their union will stop a bloody conflict that has been going on for years. However, Elaan is spoiled and unhappy with the forced marriage and acts up. While a saboteur stalks the Enterprise and a ghost sensor image appears behind them, matters are further complicated when it's revealed Elasian women can compel the love of any man... and Elaan sets her sights on Kirk.

The TARDIS is in need of repair, but when the Doctor tries to track down an old friend for help he falls into the hands of Tobias Vaughn, the megalomaniac CEO of International Electromatics. The electronics company is under observation by the United Nations Intelligence Taskforce, a new international military team for fighting threats from beyond the Earth - headed by the Doctor and Jamie's old friend, Alistair Gordon Lethbridge-Stewart (promoted to Brigadier following the Yeti invasion). Vaughn is in league with aliens who intend to invade Earth, but he intends to betray them and seize control; however, as insurance should his plans fail, he wants the Doctor's TARDIS. When Vaughn kidnaps Zoe, the Doctor requests UNIT's help to rescue her - but their interference forces Vaughn to advance the invasion schedule, leaving the Doctor with little time to mount a defense... and the situation becomes more urgent when the Doctor realises that Vaughn's allies are the Cybermen, and that their army is already in place in the sewers beneath London, waiting for the signal to attack.