  We present a {\it nonequilibrium nonperturbative} field theory for the Kondo
effect in strongly interacting quantum dots at finite temperatures. Unifying
the slave-boson representation with the Keldysh field integral an effective
Keldysh action is derived and explored in the vicinity of the zero
slave-bosonic field configuration. The theory properly reflects the essential
features of the Kondo physics and at the same time significantly simplifies a
field-theoretic treatment of the phenomenon, avoiding complicated saddle point
analysis or 1/N expansions, used so far. Importantly, our theory admits a {\it
closed analytical} solution which explains the mechanism of the Kondo effect in
terms of an interplay between the real and imaginary parts of the slave-bosonic
self-energy. It thus provides a convenient nonperturbative building block,
playing the role of a "free propagator", for more advanced theories. We finally
demonstrate that already this simplest possible field theory is able to
correctly reproduce experimental data on the Kondo peak observed in the
differential conductance, correctly predicts the Kondo temperature and, within
its applicability range, has the same universal temperature dependence of the
conductance as the one obtained in numerical renormalization group
calculations.
