A short tale which provides some remarkably accurate background to how the TARDIS works, many years before the Doctor explained it to Leela in The Robots of Death.  There's also a lovely image of the ship's interior.  A rather lovely pause in the rollercoaster of adventures presented by Chad Valley.

Decided to do all 16 stories of these I think there basically Ian 1st and Barbara stories the final 2 retell the webplanet with just those 3 but over all there not the best but I think if I was from the 1960s I’d love them