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On Your Guard! is a 1988 annual story.
Bill and Ben have been warned to be on their guard, but do not understand what it means. Ben hears some noises overnight and sees a dark shadow. It turns out to be some thieves and they are shocked when they go between a door in the shed and see what they think is the same engine. After going back and forth between the twins for a while, the thieves make a run for it. Bill and Ben tell their drivers about it the next day and say they think the thieves were trying to play hide and seek with them. Their crews are sceptical about the twins' claims.