We present the first radiative transfer (RT) model of a non-edge-on disk galaxy in which the large-scale geometry of stars and dust is self-consistently derived through fitting of multiwavelength imaging observations from the UV to the submm. To this end we used the axi-symmetric RT model of Popescu et al. and a new methodology for deriving geometrical parameters, and applied this to decode the{spectral energy distribution (SED) of M33. We successfully account for both the spatial and spectral energy distribution, with residuals typically within $7%$ in the profiles of surface brightness and within $8%$ in the spatially-integrated SED. We predict well the energy balance between absorption and re-emission by dust, with no need to invoke modified grain properties, and we find no submm emission that is in excess of our model predictions. We calculate that $80pm8%$ of the dust heating is powered by the young stellar populations. We identify several morphological components in M33, a nuclear, an inner, a main and an outer disc, showing a monotonic trend in decreasing star-formation surface-density ($Sigma_{rm SFR}$) from the nuclear to the outer disc. In relation to surface density of stellar mass, the $Sigma_{rm SFR}$ of these components define a steeper relation than the main sequence of star-forming galaxies, which we call a structurally resolved main sequence. Either environmental or stellar feedback mechanisms could explain the slope of the newly defined sequence. We find the star-formation rate to be ${rm SFR}=0.28^{+0.02}_{-0.01}{rm M}_{odot}{rm yr}^{-1}$.
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