  (abridged) A detailed study of the blue supergiant UIT005 (B2-2.5Ia+) in M33
is presented. The results of our quantitative spectral analysis indicate that
the star is a very luminous, log(L/Lsun)~5.9 dex, and massive, M~50 Msun,
object, showing a very high nitrogen-to-oxygen ratio in its surface (N/O~8, by
mass). Based on the derived Mg and Si abundances, we argue that this high N/O
ratio cannot be the result of an initial low O content due to its location on
the disk of M33, known to present a steep metallicity gradient. In combination
with the He abundance, the most plausible interpretation is that UIT005 is in
an advanced stage of evolution, showing in its surface N enrichment and O
depletion resulting from mixing with CNO processed material from the stellar
interior. A comparison with the predictions of current stellar evolutionary
models indicates that there are significant discrepancies, in particular with
regard to the degree of chemical processing, with the models predicting a much
lower degree of O depletion than observed. At the same time, the mass-loss rate
derived in our analysis is an order of magnitude lower than the values
considered in the evolutionary calculations. Based on a study of the
surrounding stellar population and the nearby cluster NGC588, using WFPC2
photometry, we suggest that UIT005 could be in fact a runaway star from this
cluster.
