Fluid dynamics of a radially spreading liquid film originated by an ideal jet that falls onto a horizontal plate are studied approximately. Five regions of different hydrodynamic structures can be singled out here. The first one is that of the normal impingement of the jet against the plate, in which the flow essentially changes its direction. The second and the third regions correspond to laminar film flow before and after the emergence of the viscous boundary layer on the free surface of the film, respectively. The fourth region represents a zone in which a hydraulic jump takes place, where the film thickness drastically increases, and the fifth one is a region of calm gravitational spreading of the film up to the liquid running off the plate. Flow patterns within all the regions except that of hydraulic jump are considered on a basis of the Karman-Pohlhausen and Blasius methods and are conjugated in between. It is shown for the first time that the hydraulic jump on a sufficiently extended film owes its origin to the fact that the region with the viscous film flow induced by the initial jet momentum must come into contact with the region of the film which spreads under gravity. The results are obtained in a simple explicit form. They may lay a foundation for heat and mass transfer studies. A transfer problem is considered within the scope of the Karman Pohlhausen method at an arbitrary Peclet number and asymptotically at high Peclet numbers with the help of the thin diffusional layer approximation.