Charlie Chan and son, Barry, in Brussels, Belgium, meet members of a group of tourists on an international tour.  On disgruntled member of the group, Mrs. Remington, is seen berating the tour organizer, Professor Ambrose, a professor at an American college, for anything that has gone awry, including the momentary loss of her purse.  As Mrs. Remington takes out her anger on Mary Ann, her personal secretary, the brusque Max Willis comes to her defense, later muttering, "One of these days I’ll murder that dame."  Barry hits it off with Sharon Prince, a young member of the tour group.  Sharon invites Barry along for a sightseeing tour and Charlie Chan agrees to the idea, saying that he will arrange for Barry and himself to join with the tour for the afternoon.

At the Congo Museum, Max Willis proves himself to be a vast repository of knowledge contrary to his apparent rough edges, as is seen as he corrects Professor Ambrose as to a fact of history during the museum tour.  Mary Ann, who has taken an obvious liking to Max, is forbidden by Mrs. Remington to see him.  The members of the tour group then go their own way, much to the frustration of Professor Ambrose.  As Chan explores the museum, there is a sudden scream followed by the sight of Mrs. Remington tumbling down a flight of stairs.  Mary Ann hastily follows down the stairs, obviously shocked by what has just happened.  As Barry and others arrive, Chan pronounces Mrs. Remington dead as Mary Ann faints in Barry's arms.  Chan mentions that before Mrs. Remington died, she whispered to him that someone had pushed her down the stairs.

Later, interviewing each of the members of the tour party, Inspector Vernet of the Brussels Police hears Professor Ambrose tell of Mrs. Remington's abusive behavior toward Mary Ann Edwards.  It comes out through Ambrose that Miss Edwards had recently been under care in a mental hospital.  It appears to the inspector that it is a simple case of a mentally unstable Mary Ann who was pushed too far by her overbearing employer.  Chan, doubtful, asks to see Mrs. Pendleton's purse.  Two things interest Chan.  First, a part of a drawstring, found in Mrs. Remington's hand, that had gone with her jewelry bag as seen earlier by Chan at the hotel as Mrs. Remington had checked the contents of her handbag.  Second, Chan notes that the jewelry bag is missing from Mrs. Remington’s purse.  Chan theorizes that Mrs. Remington died as a result of a struggle at the top of the stairs with a thief who was after the jewels which Mrs. Remington made no secret of carrying with her.  During the struggle Mrs. Remington was pushed down the stairs to her death.  Barry then notes the information he saw in a letter he had found earlier.  No longer in Barry's pocket, Max Willis offers it up, mentioning his earlier career as a pickpocket.  Max had written the letter to an old cellmate of his at Sing Sing prison where Max had served time for armed robbery.  Max then hurriedly confesses to the theft of the jewels and the murder of Mrs. Remington.

Minutes later as Vernet rounds up the tour members, Chan rushes in asking the whereabouts of Mary Ann.  Vernet tells him that she became hysterical and that Professor Ambrose has taken her into the museum's garden to calm her down.  Chan then states that they must find them immediately.  In the garden, Ambrose walks a calmed Mary Ann across a bridge spanning a large peaceful pond.  Thanking the professor, Mary Ann suggests it is getting late and that they should return to the tour group.  Ambrose then reminds Mary Ann of her fragile mental health, seeking to convince her that it was she who pushed Mrs. Remington down the stairs to her death and that the best way out for her would be to commit suicide my drowning herself in the pond.  Suddenly, Mary Ann remembers with clarity that she was not with Mrs. Remington at the crucial moment as she had moved off momentarily to look at an exhibit.  Hearing approaching voices calling out to Mary Ann, Professor Ambrose subdues Miss Edwards in an attempt to kill her before the search party arrives.  Seeing the violent struggle on the bridge, Chan and the approaching group hurry to the scene as Ambrose is about to push Mary Ann into the water below.  Chan pulls his gun and wounds Ambrose who drops Mary Ann.  Chan then correctly suggests that the missing jewelry is on the professor's person. 

As Mary Ann and Max leave in each other's arms, Chan notes to Barry two things that pointed to Ambrose as the possible murderer: first, Ambrose's ignorance of a very obvious fact of history and second, the phone call to America revealed non-existence of the college where Ambrose claimed to be a professor.

Ancient wisdom says, "Anger very bad for digestion."

Wise old Chinese friend one said, "Two birds on branch sing sweetly, but third one changes melody to discord."