relationship of TEM wave in free space and parallel plates?

relationship of TEM wave in free space and parallel plates?
TEM wave can transmit in free space and also can be in parallel plates.
However, why the TEM wave can not still transmit when a pair of parallel plates open end(open circuit) and it totally reflect?
I think the free space can support the TEM wave.

This problem confuse me sevral years.
any help is welcome
thanks in advance.

To venture a guess, maybe it has to do with a mismatch in the impedance* between the two 'waveguides' ?

*(and related aspects of how the mode is sustained and propagated)