Christopher Race, forced by his uncle to earn his own living for a change, buys Scarlet Runner, an exceptionally fine automobile. He at first tries hiring it out to private parties, but meets with no success. Finally, after a warning from his uncle that he must earn a stated sum by the end of the year or suffer disinheritance, he turns his car into a public conveyance and tours the streets. He finds the sovereign of a foreign power in a broken-down car and carries him to his destination. By means of his car, and its terrific speed, he is able to thwart a plot which nearly causes the death of Martin Linden, who is a friend of Christopher's and who is engaged to marry a relative of the latter's, Lady Ivy. He causes the destruction of the plotters and the happy reunion of the lovers, all by his red car and his ready wit. He earns the stated sum, and more, too, and then fares forth once more in search of further adventure.