  AIMS: We selected the bluest object, WISE~J0725$-$2351, from Luhman's new
high proper motion (HPM) survey based on observations with the Wide-field
Infrared Survey Explorer (WISE) for spectroscopic follow-up observations. Our
aim was to unravel the nature of this relatively bright ($V$$\sim$12,
$J$$\sim$11) HPM star ($\mu$$=$267\,mas/yr). METHODS: We obtained low- and
medium-resolution spectra with the European Southern Observatory (ESO) New
Technology Telescope (NTT)/EFOSC2 and Very Large Telescope (VLT)/XSHOOTER
instruments, investigated the radial velocity and performed a quantitative
spectral analysis that allowed us to determine physical parameters. The fit of
the spectral energy distribution based on the available photometry to
low-metallicity model spectra and the similarity of our target to a metal-poor
benchmark star (HD~84937) allowed us to estimate the distance and space
velocity. RESULTS: As in the case of HD~84937, we classified WISE~J0725$-$2351
as sdF5: or a metal-poor turnoff star with $[Fe/H]$$=$$-$2.0$\pm$0.2,
$T_{eff}$$=$6250$\pm$100\,K, $\log{g}$$=$4.0$\pm$0.2, and a possible age of
about 12\,Gyr. At an estimated distance of more than 400\,pc, its proper motion
translates to a tangential velocity of more than 500\,km/s. Together with its
constant (on timescales of hours, days, and months) and large radial velocity
(about $+$240\,km/s), the resulting Galactic restframe velocity is about
460\,km/s, implying a bound retrograde orbit for this extreme halo object that
currently crosses the Galactic plane at high speed.
