  The Galactic TeV $\gamma$-ray source HESS$\,$J1804$-$216 is currently an
unidentified source. In an attempt to unveil its origin, we present here the
most detailed study of interstellar gas using data from the Mopra Southern
Galactic Plane CO Survey, 7 and 12$\,$mm wavelength Mopra surveys and Southern
Galactic Plane Survey of HI. Several components of atomic and molecular gas are
found to overlap HESS$\,$J1804$-$216 at various velocities along the line of
sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both
correlation and anti-correlation between the gas and TeV $\gamma$-ray emission
have been identified in various gas tracers, enabling several origin scenarios
for the TeV $\gamma$-ray emission from HESS$\,$J1804$-$216. For a hadronic
scenario, SNR$\,$G8.7$-$0.1 and the progenitor SNR of PSR$\,$J1803$-$2137
require cosmic ray (CR) enhancement factors of $\mathord{\sim} 50$ times the
solar neighbour CR flux value to produce the TeV $\gamma$-ray emission.
Assuming an isotropic diffusion model, CRs from both these SNRs require a slow
diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM
gas. The morphology of gas located at 3.8$\,$kpc (the dispersion measure
distance to PSR$\,$J1803$-$2137) tends to anti-correlate with features of the
TeV emission from HESS$\,$J1804$-$216, making the leptonic scenario possible.
Both pure hadronic and pure leptonic scenarios thus remain plausible.
