  We report simultaneous photometric and echelle-spectroscopic observations of
the dwarf nova BZ UMa during which we were lucky to catch the system at the
onset of an outburst, the development of which we traced in detail from
quiescence to early decline. The outburst had a precursor, and was of a short
duration (~5 days) with a highly asymmetrical light curve. On the rise we
observed a `jump' during which the brightness almost doubled over the course of
half an hour. Power spectra analysis revealed well-defined oscillations with
period of ~42 minutes. Using Doppler tomography we found that the unusual
emission distribution detected in quiescence held during the outburst. After
the maximum a new emission source arose, from the inner hemisphere of the
secondary star, which became the brightest at that time. We analyse this
outburst in terms of `inside-out' and `outside-in' types, in order to determine
which of these types occured in BZ UMa.
