  The OPERA neutrino detector in the underground Gran Sasso Laboratory (LNGS)
has been designed to perform the first detection of neutrino oscillations in
direct appearance mode through the study of the $\nu_\mu\rightarrow\nu_\tau$
channel. The hybrid apparatus consists of an emulsion/lead target complemented
by electronic detectors and it is placed in the high energy long-baseline CERN
to LNGS beam (CNGS) 730 km away from the neutrino source. Runs with CNGS
neutrinos were successfully carried out in 2008 and 2009. After a brief
description of the beam, the experimental setup and the procedures used for the
analysis of the neutrino events, we describe the topology and kinematics of a
first candidate $\nu_\tau$ charged-current event satisfying the kinematical
selection criteria. The background calculations and their cross-check are
explained in detail and the significance of the event is assessed.
