Colonel and Mrs. Wagstaff and their daughter Susan take the 'Bull and Cow' hotel near Seldon Park racecourse where they hope to live comfortably and profitably. Unfortunately their first guests are Alfred Tubbe and his runner Fred Phipps, and a subterranean guest, the unknown third of their party, 'Flash' Harry. It is the purpose of these three to switch the favourite in the local big race with a horse which is concealed with 'Flash' in the subterranean passages that tunnel the ground beneath the old hotel.