The formation of periodic plasma clots which form a discharge channel similar to bead lightning has been studied with a four-channel intensified charge-coupled device (ICCD) camera simultaneously with the recording of voltage and current waveforms. It was found that a diffuse discharge was formed at the initial stage due to the development of a wide streamer. Runaway electrons were registered. Beads were formed as a result of the development of the spark leader after the diffuse stage. It was assumed that the diameter of the spark leader periodically changes in accordance with the change in the electric field strength in the spark leader head. As a result, narrow sections of the channel heat up more and glow brighter.