  Long gamma-ray bursts give us the chance to study both their extreme physics
and the star-forming galaxies in which they form. GRB 100418A, at a z = 0.6239,
had a bright optical and radio afterglow, and a luminous star-forming host
galaxy. This allowed us to study the radiation of the explosion as well as the
interstellar medium of the host both in absorption and emission. We collected
photometric data from radio to X-ray wavelengths to study the evolution of the
afterglow and the contribution of a possible supernova and three X-shooter
spectra obtained during the first 60 hr. The light curve shows a very fast
optical rebrightening, with an amplitude of 3 magnitudes, starting 2.4 hr after
the GRB onset. This cannot be explained by a standard external shock model and
requires other contributions, such as late central-engine activity. Two weeks
after the burst we detect an excess in the light curve consistent with a SN
with peak absolute magnitude M_V = -18.5 mag, among the faintest GRB-SNe
detected to date. The host galaxy shows two components in emission, with
velocities differing by 130 km s^-1, but otherwise having similar properties.
While some absorption and emission components coincide, the absorbing gas spans
much higher velocities, indicating the presence of gas beyond the star-forming
regions. The host has a star-formation rate of 12.2 M_sol yr^-1, a metallicity
of 12 + log(O/H) = 8.55 and a mass of 1.6x10^9 M_sol. GRB 100418A is a member
of a class of afterglow light curves which show a steep rebrightening in the
optical during the first day, which cannot be explained by traditional models.
Its very faint associated SN shows that GRB-SNe can have a larger dispersion in
luminosities than previously seen. Furthermore, we have obtained a complete
view of the host of GRB 100418A owing to its spectrum, which contains a
remarkable number of both emission and absorption lines.
