  Cold neutral gas is a key ingredient for growing the stellar and central
black hole mass in galaxies throughout cosmic history. We have used the Atacama
Large Millimetre Array (ALMA) to detect a rare example of redshifted
$^{12}$CO(2-1) absorption in PKS B1740-517, a young ($t \sim 1.6 \times 10^{3}$
yr) and luminous ($L_{\rm 5 GHz} \sim 6.6 \times 10^{43}$ erg s$^{-1}$ ) radio
galaxy at $z = 0.44$ that is undergoing a tidal interaction with at least one
lower-mass companion. The coincident HI 21-cm and molecular absorption have
very similar line profiles and reveal a reservoir of cold gas ($M_{\rm gas}
\sim 10^{7} - 10^{8}$ M$_{\odot}$), likely distributed in a disc or ring within
a few kiloparsecs of the nucleus. A separate HI component is kinematically
distinct and has a very narrow line width ($\Delta{v}_{\rm FWHM} \lesssim 5$ km
s$^{-1}$), consistent with a single diffuse cloud of cold ($T_{\rm k} \sim 100$
K) atomic gas. The $^{12}$CO(2-1) absorption is not associated with this
component, which suggests that the cloud is either much smaller than 100 pc
along our sight-line and/or located in low-metallicity gas that was possibly
tidally stripped from the companion. We argue that the gas reservoir in PKS
B1740-517 may have accreted onto the host galaxy $\sim$50 Myr before the young
radio AGN was triggered, but has only recently reached the nucleus. This is
consistent with the paradigm that powerful luminous radio galaxies are
triggered by minor mergers and interactions with low-mass satellites and
represent a brief, possibly recurrent, active phase in the life cycle of
massive early type galaxies.
