The background is split into sections by diagonal zig-zag lines. Each of these bkg panels shows a different time period – The Cretaceous Period (T-Rex), Roman times (Centurions), the 80’s (T-Rex), the future (jetpacks).
In the foreground, existing in all or flitting between these time periods, we see the Back to the Future DeLorean crashed into the side of the TARDIS.
The Doctor scans the DeLorean with the sonic screwdriver.
Doc Brown pokes his head through the TARDIS doors.
The Doctor: Reverse the polarity of the neutron flow?
Doc Brown: Precisely!