Synopsis: Apparently, the Time Lords aren't the only ones capable of meting out justice for temporal incursions. On Nefrin, aeons ago, a woman named Brimo was imprisoned in an Eternity Capsule for trying to subert the course of time.
Millions of years pass and Nefrin's star collapses, drawing every planet in the system into a Black Hole. The force of that much gravity releases Brimo into a whole new reality, where every wish is granted. The only problem is that the fuel for this pocket universe comes from "our reality", where a rift has now appeared, sucking vast amounts of energy from our universe to Brimo's.
The TARDIS passes by this rift, gets stuck, and effectively closes the rift. Brimo senses the change in the order of things, focuses in on the TARDIS, and draws the Doctor into combat, with the hope of dislodging the TARDIS and returning power to her universe.
Since it is quite literally a battle of wits, though, Brimo soon finds herself outmatched. In the end the Doctor asks her to imagine the worst thing possible. She naturally thinks of being imprisoned by the Eternity Capsule. But since this is a universe in which thought becomes reality, she is once again trapped in the Eternity Capsule. Victorious, the Doctor rush back to the the TARDIS, planning on a victorious dematerialization before Brimo's imaginary world completely collapses.
Things aren't quite so simple, though. Half of the TARDIS is now outside the bounds of Brimo's collapsing reality. Unfortunately, the chronal compensators (that part of the TARDIS which neutralizes the aging of its inhabitants as they pass through time) are on the other side of the rift. The only way to survive is an emergency dematerialization to force the TARDIS to reintegrate itself. When he does this, the Doctor discovers that the two halves of the TARDIS were four years apart from each other. Without the compensators, the TARDIS occupants are forced to accept the chronology of the half that had already passed out of Brimo's reality. He turns around to discover that Sharon is no longer a schoolgirl but a young woman!
Notes: Reprinted from Doctor Who Magazine (1979) #35-38