The auto guard was an automatic sentry device installed in the control room of Space Station Nerva or "the Ark". It was designed to lock onto any movement made by an organic substance, such as a human, and fire electrical bolts at it. These bolts carried a charge of 500,000 volts and were powerful enough to kill almost anything.
When the Wirrn Queen infiltrated Nerva, the auto guard shot her with an electrical bolt. The Queen managed to disable the Auto guard before she died by sabotaging the station's control circuitry and completed her task of impregnating a cryogenic sleeper with her eggs, succumbing to her wounds further into the station. When the Fourth Doctor later repaired the circuitry to restore power to the station's oxygen pumps and doors, he inadvertently re-activated the Auto guard as well. He and Harry ducked behind a desk to avoid the blasts, sacrificing along the way: both of Harry's shoes, the end of the Doctor's scarf, part of his hat and a cricket ball. Eventually they moved the desk over to the auto guard's cut-off switch and used Harry's second shoe to draw its fire while the Doctor turned off the power. (TV: The Ark in Space)