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The formation and dynamical evolution of cometary plasma tails and magnetic boundary layers is studied by the first numerical plasma-neutral gas simulations. It is shown that collisional interaction between the cometary neutral envelope and the solar wind plasma leads to the formation of a magnetic barrier that separates the entire cometary body from the solar wind. The overall dynamics, in particular, of the magnetotail are governed by magnetic reconnection. Multiple reconnection events are initiated by the localized excitation of anomalous resistivity modeled by η(j). If the comet encounters a heliospheric current sheet, strong disconnection events characterize the cometary plasma tail. But even in the case of homogeneous solar wind conditions, partial disruption of the tail is triggered by dayside reconnection.