The Mystery of Mr. Wong begins as Brandon Edwards drifts alone in a small boat in the San Francisco Bay. When a large passenger boat passes by, an unknown Asian man throws a tiny package into the water. Edwards retrieves it, and shoves it into his jacket for safe keeping. As his boat docks, we see an unknown Asian man keeping tabs on Edwards from the shadows.
Edwards then returns home in a Yellow Cab after being out all night, and rushes upstairs to his den. Meanwhile, in another room, his wife, Valerie, is with his secretary Peter Harrison. Harrison knows that Brandon terrifies Valerie, and even though she tries to hide it. it angers him greatly.
As Edwards cuts open the package he retrieved from the water, his maid, Drina, who is an illegal Chinese immigrant, runs to another servant, Michael Strogonoff who’s playing the piano. Drina reveals that the two are at the Edwards house for some unknown mission, but that Strogonoff is jeopardizing it because he’s in love with Valerie.
Back in Valerie’s room, Harrison reveals he’s in love with her too. As Strogonoff sings away downstairs, Harrison grabs Valerie, and tells her that he can not stand to see her in pain. Someday, he will kill Edwards.
Edwards smugly walks into the room, looks at Harrison, and gives Valerie a kiss on the cheek. He asks her if she’s completed the guest list for that evening’s party because he wants to make sure that she’s invited Prof. Ed Janney so he can show him something. He also wants to make sure a Chinese detective named Mr. James Lee Wong attends.
Edwards, annoyed with Strogonoff’s singing, has Harrison go downstairs to tell him to stop. Then he accuses Valerie of having other reasons for Strogonoff moving in with them, and tells her he’s not blind. He accuses her of carrying on an affair with both Strogonoff and Harrison.
While she feigns offense at the accusation, Edwards smirks with a turn. He then reveals that last night while he was out, he’s just come into possession of the Eye of the Daughter of the Moon, the largest star sapphire in the world. Coincidentally, someone has just stolen that very gem in China. I’m not naming any names though.
Valerie is horrified to learn he’s brought it into the house. The Emperor Hong Chong Tu placed a curse upon it as he buried it in his dead, unfaithful wife’s heart. Now, Edwards’ unfaithful wife fears they all will soon be dead.
Hours later, night falls, and the party begins. Mr Wong arrives to the event inexcusably late, but Edwards happily greets him, as does Janney. Edwards brings him upstairs to his den alone. There he shows Wong the gem, and a letter he received before obtaining the stone; one warning him of his imminent death.
When Wong asks him why he doesn’t just have Janney help him, Edwards replies that he doesn’t trust him either, and insinuates that Valerie is having an affair with him as well. She gets around. He then shows Wong a sealed letter in his safe to be read if anything ever happens to him. The name of the person he suspects is in it.
The two men head down to play a game of Indications (Charades) that Valerie is hosting. During the game, while the lights are low, someone shoots Edwards dead as they act out Murder comes at Midnight. It appears that Harrison killed him because he’s holding a gun in his hand, but it’s loaded with blanks. So, who shot Edwards?
The police show up unannounced because someone called to tell them a murder was committed. However, at the time of the call, Edwards was still alive. Meanwhile, Drina opens the safe in Edwards’ den, and steals the sealed letter.
As Wong and Police Capt. Sam Street begin their investigation, Sing, Edwards’ Butler reveals that his boss had a big argument with Harrison the day before. The two next question Harrison about it. He won’t talk, so, Street holds him as a material witness, and take him to jail.
Wong tells Janney he suspects foul play because of the letter which Janney didn’t know about. He instead reveals to Wong that Valerie has many admirers including Strogonoff. The two men find the safe open with the letter and the gem missing from it.
The next day, Wong analyzes the note in his possession warning of Edwards’ imminent death. He asks his butler, Willie, to go to Chinatown to the House of the Seven Brothers and talk to Sum Yung about who buys such paper that the note was written on.
At the police station, the ballistics test reveals that the bullet that killed Edwards could not have come from Harrison’s gun, and in fact was most likely from a gun with a silencer. Also, the bullet was shot from above Edwards—not straight on.
Also to add to the intrigue, Carslake, Edwards’ attorney arrives with information that Edwards was in the process of changing his will to disinherit his cheating wife as much as possible. Carslake reveals that Edwards’ butler, Sing would have now been the biggest recipient of his fortune. With this new news, the police release Harrison from their custody, and they all head back to Edwards’ mansion. It’s time to question everyone since everyone is now a suspect.
That night, Wong and Janney spend another night at the mansion. Sing opens a window in the den because Wong is smoking. Wong then questions him, and says he knows that Sing is hiding something.
Meanwhile, Strogonoff meets outside with Drina in secret. Drina asks him where he hid the gem, but he says he doesn’t have it. Then she gets mad at him for not marrying her like he promised. She thinks he’s holding out on her, and she tells him that he must be the one in Edwards’ letter. As this is going on, we see someone lurking in the shadows. Angry, Drina then threatens to mail the letter to Wong before rushing off in a huff.
Back in her room, she puts the letter in a new envelope addressed to Wong while she nervously puffs away on a cigarette. She then stashes the letter behind a painting, and plops down on her bed. Later, one of the police guards stationed at the mansion finds the window that Sing left wide open with a ladder propped outside of it.
With some unknown killer in the house, the cops wake everyone. Everyone shows, but Drina as she’s dead in her room. Inside it, Wong finds the torn remains of the envelope the original letter was in and a half-smoked cigarette. Later, as he investigates the ladder outside, someone takes a shot at him with a gun that has a silencer attached.
Don’t worry. The shooter missed, and Wong returns inside to find Strogonoff and Harrison chatting atthe base of the staircase. He bums a cigarette off both men, and retires to the den where he glues the envelope together. As he finishes, Janney shows up, and Wong reveals that he thinks Drina knew who killed Edwards, and the killer silenced her forever.
Captain Street shows up to berate his men for letting someone kill Drina. Sing says he saw Drina talking outside to Strogonoff before her death. Strogonoff retorts that they were merely talking about personal matters. Street then arrests Strogonoff before Wong reveals someone shot at him just minutes earlier.
The next morning, Sing finds the letter behind the painting in Drina’s room, and calls Wong to tell him. Wong is at home where Willie has returned with a list of two people who may have purchased that paper the death threat was written on. As he gives the list to Wong, Sing calls. However, Sing can not get out what he has to say before someone comes up from behind, and knocks him out cold. When no one answers Wong’s return call, he calls Captain Street. The two men then rush off to the Edwards mansion.
They break in when no one comes to the door to find Sing face down on the ground next to the phone. While they talk in the parlor, Valerie, Harrison, and Janney return from a thirty minute walk. Street questions them all more before Wong lets it slip that Sing already put the letter in the mail.
That night at Wong’s residence, the suspects all begin arriving one after another—wanting to know the contents of the letter. Valerie, then Harrison, Janney, and Strogonoff enter. Strogonoff begs Wong not to open the letter, but he rsimply eplies that Strogonoff is not his real name. He knows that he and Drina planned to steal the gem from Edwards all along; even if it meant killing him for it.
Wong has Strogonoff to have a seat as Carlake shows up. He’s still nervous that he’s a suspect. Wong sits them all down, and shows them the unopened incriminating envelope. He still doesn’t open it though in case Edwards was wrong. He doesn’t want to put an innocent person in harms way.
So he begins monologuing; eliminating the murder possibilities one by one. He eliminates the theft of the gem as the motive for murder because his local Chinamen assure him that’s not a possibility. Wong then eliminates Strogonoff as a suspect to steal the gem as he never had the opportunity with the gem locked away.
The revised will is next eliminated as a motive for murder as the two who would benefit, Valerie and Harrison, were on stage at the time, and them clearly could not have killed Edwards. Carlake, while a viable accomplice, couldn’t have helped them, even though he objected to Edwards disinheriting Valerie. The police already corroborated his story of being out on a drive at the same time as the murder.
That leaves only one last person who could have done it. That person is the one written in the letter. Wong says he will not open it if Janney tells him why he killed Edwards. He knows because of Janney’s mistakes along the way. Willie easily traced the paper from the death threat back to his university where it’s used in his department to repair manuscripts. Also, when Wong discovered that the Manuscript of Woo Wang was an obvious forgery, and that Janney knew it. Since encouraged Edwards to buy it anyway, Wong knew Janney wasn’t really a friend.
As for a motive, Wong discovered that Janney’s sister was Edwards’ first wife whom he drove to suicide. Janney harbored a resentment ever since. When Janney witnessed Edwards treat Valerie the same way, and then when heard he was disinheriting her, Janney made up his mind to remove him.
As Wong reveals these details, Janney casually pulls out a cigarette. Before he can light it though, Wong stops him. Wong laments about poor Drina who died from poisoned smoke in her lungs, and grabs the cigarette out of Janney’s hand. He knows that cigarette in Janney’s hand is poisoned the same way, and prevents Janney from committing suicide.
Captain Street shows up, and Wong lets him know Janney is the killer; much to the Captain’s dismay as the three men are friends. Janney shakes Wong’s hand goodbye. As Janney parts, Wong opens his hand to reveal that Janney slipped him the stolen gem. He casually slips it in his pocket as the Captain opens the letter which is blank. The real letter hadn’t arrived yet.
After everyone leaves, a messenger delivers the letter. Wong tears it up without opening it, and sends Willie on a very important errand. He want Willie to take the gem back to China where it belongs.