High‐speed video and electric field change data were used to analyze the initiation and propagation of four predominantly vertical bidirectional leaders making connection to a predominantly horizontal channel previously formed aloft. The four bidirectional leaders sequentially developed along the same path and served to form a positive branch of the horizontal in-cloud channel, which became a downward positive leader producing a 135-kA positive cloud-to-ground (+CG) return stroke. The positive (lower) end of each bidirectional leader elongated abruptly at the time of connection of the negative (upper) end to the pre-existing channel aloft. Twenty-six negative streamer-like filaments (resembling recently reported “needles”) extended sideways over ~100 to 750 m from the pre-existing horizontal channel at speeds of ~0.5 to 1.9 ×10^7 m/s, in response to the injection of negative charge associated with the +CG.