The unearthing of an alien device enabling instantaneous space travel creates a headache for U.N.I.T. when Brigadier Alastair Gordon Lethbridge-Stewart and an exploration team go through the portal...and don't come back. Meanwhile, decades later, U.N.I.T. General Yvette Dwonch and her research team come upon the same device, utilize its properties, and also fail to return. Now their fates lie in the hands of the two Time Lords who can solve this dilemma: Maureen (the fifteenth incarnation of Neas) and the third incarnation of her old mentor and friend, the Doctor.