The Interdimensional Detective

Ava Brooks has had something strange happen to her. Her brother, River has mysteriously disappeared and left with barely any trace. All that's left are a broken window latch and a poster tacked on her apartment's bulletin board with a phone number for some "Detective".

Ava decides to take her chances and dial the number. But this "Detective" is a little bit strange...

Who is this person?
Why is she in a mobile toilet?

And, to Ava, the most pressing question.

Will they really be able to rescue River?