Professor Grimes, a crank on scientific matters, is addicted to somnambulism without the knowledge of his associates. He is impetuous and easily disturbed, consequently, he is feared by Susie Davis, his private secretary. The story opens with Grimes receiving a large sum of money from a caller. As he is inclined to be miserly, he starts for the bank to deposit the money, but finds it is closed. Returning home, he places it in the safe. Grimes has a nephew, Ralph, who is being hounded by a creditor, and he hasn't the money to liquidate the indebtedness. He tells his troubles to Susie Davis, his sweetheart, and she assists him, giving him the money she has saved through frugality. The next morning the professor misses his money when he examines the safe and is wild. He secures the services of a detective and that investigator is not long in discovering that the nephew has paid a debt with quite a large sum of money. Ralph is under suspicion. He proves his innocence by the girl to the satisfaction of the sleuth, who is in a quandary as to how the money disappeared. He resolves to keep a watch on the safe and does so. In the still hours of the night Professor Grimes slowly descends the stairs, walking in his sleep, and returns the money to the safe. The mystery is cleared up. In the morning Grimes is confronted by the detective, who explains to him the mystery of the safe robbery. Ralph and Susie are rewarded by being restored to the good graces of the uncle, who wishes them to forget all about the incident.
—Moving Picture World synopsis