Logopolis, Part 1
Written by Chris Bidmead
Directed by Peter Grimwade
The Doctor becomes obsessed with fixing the chameleon circuit. He goes to Earth to borrow a police box, which he will measure and then take the measurements to Logopolis to get the circuits programmed in so they are an exact match.
On Earth, he finds a police box sitting on the side of the road. He materializes the Tardis around it - or so he thinks.
During his travels, he witnesses a strange white-clad figure in the distance.
A young flight attendant on her first day working, has an interesting (if abrasive) personality. Her aunt volunteers to drive her to work, but the car breaks down along the way. This flight attendant, Tegan, sees a blue police box in the distance and walks over to it to call for help.
Inside she goes ... it's the Tardis.
The Doctor and Adric go inside the other police box, only to find out inside it is a Tardis, a duplicate of the Doctor's, only with a different color scheme. And in they go, and in they go. It reminds me of that episode of Star Trek: TNG where they find the Yamata, and Riker and (Worf?) go to the door on the bridge, only to look through and see themselves staring from the other side, it's an endless cycle.
It is the Master's Tardis. And while Tegan is inside, he is playing with her aunt. Yep, he shrinks her down to the size of a doll.
* * *
It's never a good thing for the companions with the Doctor becomes obsessed with something. Remember when Six becomes obsessed with atoning? He tries to kill Peri. Ah, youth.