Summary: Something us seriously wrong; the TARDIS is getting smaller.
The TARDIS is shrinking, which is more than a little unexpected. Already it’s like a half-scale model of itself, and getting smaller all the time. At first, getting in and out can just about be accomplished, although it’s a bit of a squeeze fitting through the door, but that stage doesn’t last long. The shrinking isn’t stopping. To Clara, it’s looks adorable, but to the Doctor it’s a serious problem that needs fixing as soon as possible.
With the Doctor inside and Clara outside, somehow they’ll have to work together to solve the mystery and restore the TARDIS to normal.