We present results from integral field optical spectroscopy with the Potsdam Multi-Aperture Spectrograph of the HerbigHaro (HH) object HH 204, with a spatial sampling of 1 x 1 arcsec2. We have obtained maps of different emission lines, physical conditions and ionic abundances from collisionally excited lines. The ionization structure of the object indicates that the head of the bow shock is optically thick and has developed a trapped ionization front. The density at the head is at least five times larger than in the background ionized gas. We discover a narrow arc of high Te([N ii]) values delineating the southeast edge of the head. The temperature in this zone is about 1000 K higher than in the rest of the field and should correspond to a shock-heated zone at the leading working surface of the gas flow. This is the first time this kind of feature is observed in a photoionized HH object. We find that the O+ and O abundance maps show anomalous values at separate areas of the bow shock probably due to: (a) overestimation of the collisional de-excitation effects of the [O ii] lines in the compressed gas at the head of the bow shock, and (b) the use of a too high Te([N ii]) at the area of the leading working surface of the flow.