  We present the first detailed analysis of the rest-frame UV spectrum of the
gravitationally lensed Lyman break galaxy (LBG), the `8 o'clock arc'. The
spectrum of the 8 o'clock arc is rich in stellar and interstellar medium (ISM)
features, and presents several similarities to the well-known MS1512-cB58 LBG.
The stellar photospheric absorption lines allowed us to constrain the systemic
redshift, z_sys = 2.7350+/-0.0003, of the galaxy, and derive its stellar
metallicity, Z=0.82 Z_sol. With a total stellar mass of ~4.2x10^{11} M_sol, the
8 o'clock arc agrees with the mass-metallicity relation found for z>2
star-forming galaxies. The 31 ISM absorption lines detected led to the
abundance measurements of 9 elements. The metallicity of the ISM, Z=0.65 Z_sol
(Si), is very comparable to the metallicity of stars and ionized gas, and
suggests that the ISM of the 8 o'clock arc has been rapidly polluted and
enriched by ejecta of OB stars. The ISM lines extend over ~1000 km/s and have
their peak optical depth blueshifted relative to the stars, implying gas
outflows of about -120 km/s. The Ly-alpha line is dominated by a damped
absorption profile on top of which is superposed a weak emission, redshifted
relative to the ISM lines by about +690 km/s and resulting from multiply
backscattered Ly-alpha photons emitted in the HII region surrounded by the
cold, expanding ISM shell. A homogeneous spherical radiation transfer shell
model with a constant outflow velocity, determined by the observations, is able
to reproduce the observed Ly-alpha line profile and dust content. These results
fully support the scenario proposed earlier, where the diversity of Ly-alpha
line profiles in LBGs and Ly-alpha emitters, from absorption to emission, is
mostly due to variations of HI column density and dust content (abridged).
