It is assumed that bag film disintegration is governed by two factors: liquid gathering into a moving toroidal roller due to the surface tension forces, and periodic splashings of the roller under the action of film instability. After the initial film perforation the surface tension grasps a significant amount of liquid into the roller before an unstable disturbance shakes it off from the edge. A solution of the simplest instability problem for a liquid layer in gas flow was applied to determine the characteristic size and time of the dominant disturbance. Using the simplest assumptions (media are ideal, film is plane, liquid roller has a form of torus) the differential equation of a roller motion under the action of surface tension forces and reactive forces is derived, and the laws of roller motion and growth are calculated. Taking into account simultaneously both mechanisms of unstable shaking-off and of the liquid roller growth, the main regularities of the film disintegration kinetics have been calculated: histories of the quantity of shaken-off droplets and of the roller radius, the number distributions of droplets, and the period of whole film disintegration.