Models of the chemical evolution of the Milky Way suggest that: the observe
d abundances of elements heavier than helium ('metals') require a continuou
s infall of gas with metallicity (metal abundance) about 0.1 times the sola
r value. An infall rate integrated over the entire disk of the Milky Way of
similar to 1 solar mass per year can solve the 'G-dwarf problem'-the obser
vational fact that the metallicities of most long-lived stars near the Sun
lie in a relatively narrow range(1-3). This infall dilutes the enrichment a
rising from the production of heavy elements in stars, and thereby prevents
the metallicity of the interstellar medium from increasing steadily with t
ime. However, in other spiral galaxies, the low-metallicity gas needed to p
rovide this infall has been observed only in associated dwarf galaxies(4) a
nd in the extreme outer disk of the Milky Way(5,6). In the distant Universe
, low-metallicity hydrogen clouds (known as 'damped Ly alpha absorbers') ar
e sometimes seen near galaxies(7,8). Here we report a metallicity of 0.09 t
imes solar for a massive cloud that is falling into the disk of the Milky W
ay. The mass now associated with this cloud represents an infall per unit a
rea of about the theoretically expected rate, and similar to 0.1-0.2 times
the amount required for the whole Galaxy.