A 500 bank note is stolen from Lady Lydiard. Present in the household are Isabel Miller, her adopted daughter; Felix Sweetsir, her nephew; Robert Moody, the steward; and Alfred Hardyman who has come to advise about Tommie, the ailing Scotch Terrier. Suspicion rests on Isabel who goes to stay with her aunt until her name is cleared. Moody, in love with Isabel, engages the slovenly detective, Old Sharon, to find the real thief. In the country at South Mordern, Isabel once again meets Hardyman who proposes marriage. She accepts on condition that Hardyman's family and friends will accept her. On the day of a celebratory luncheon they cancel because Felix, the real thief, has leaked the news that Isabel is suspected. In fact Felix stole the bank note to repay a gambling debt to Hardyman. The note is in Hardyman's missing pocket book, discovered by the gallant Tommie playing canine detective. Felix makes a lame excuse to Lady Lydiard before escaping to the continent. Isabel agrees to marry the faithful Moody."