At the end of one of detective Mr. Moto's classes in criminology, student Lee Chan, the son of the famous Honolulu detective Charlie Chan, notices that his watch has been stolen. Horace "Knockout" Wellington, another student, confesses that he is the culprit and reveals, to the mirth of all, that he cannot help taking things that attract his eye and later cannot remember where he purloined the objects. Moto invites Lee to accompany him and Lieutenant Riggs to a boxing match between Bill Steele and Frankie Stanton to decide who will fight world champion Biff Moran eight weeks later. Before the fight, gambler Nick Crowder bets $10,000 with bookie Clipper McCoy that Stanton will not last until the fifth round. Because he has already taken similar bets from gamblers in six other cities, Clipper warns Stanton's manager, Jerry Connors, that Stanton better not throw the fight. Riggs, Moto and Lee sit with Philip Benton, whose company owns the arena, and Benton's snooty daughter Linda, who is interested in Bill even though he likes sports reporter Penny Kendall. After Stanton's eye is cut, the referee threatens to stop the fight. Connors applies collodion to close the wound, and in the next round, Bill knocks Stanton out. When the doctor pronounces Stanton dead, Moto recovers a dried bit of collodion. Connors, who threw the bottle he used out a window, gives Moto another bottle to examine. Benton, who earlier told Riggs that he never bets, calls Clipper, who has lost $130,000 on the fight, to say that he will send a check for $10,000 to cover his bet. The poison that killed Stanton is found on Bill's gloves, and he is charged with manslaughter and suspended by the boxing commission. Although Penny convinces her editor to put up the bond money, she finds Bill leaving jail with Linda, who has already paid with a check from her father. When Moto proves to his class that the poison was shot onto Bill's glove from someone outside the ring, Lee and Wellington leave to investigate. Moto finds that Wellington has taken his overcoat and left one he earlier took from the arena, which has a stain on the inside that matches the stain found on Bill's glove. Meanwhile, at Moran's training camp, Benton and press agent Gabby Marden overhear Clipper accuse Nick of placing bets in other cities to keep the odds up at the fight. As Gabby goes to call Riggs with the information, someone takes a shot at him. After Riggs and Moto find that the owner of the overcoat, John Howard, has died from the same poison that killed Stanton, Moto, believing that Howard was a fall guy, convinces the boxing commission to lift its suspension of Bill and promises to have the murderer arrested at ringside the night of Bill's fight with Moran. In his room, Moto is confronted with an armed man, but Moto's cat Lena knocks over a vase, which allows Moto to grab the gun before the intruder escapes. At the vacant arena, a mysterious figure places a gun attached to a clock under the ring, and aims it at Moto's seat. Before the fight, Penny kisses Bill and wishes him well and then berates Linda for only caring for a winner. With the gun pointed at him, Moto surveys his suspects as the fight proceeds. He then invites Linda to sit in his seat, and after Bill knocks out Moran, Moto announces that he will shortly produce Stanton's murderer. When he insists, despite Benton's attempt to get his daughter away, that Linda remain in the seat, Benton rushes to the ring and disconnects the gun before it fires. Moto explains that he investigated the arena before the fight, because he knew that the murderer would try to kill him to keep him from revealing his identity, and he took the bullets out of the gun. As Penny hugs Linda, Benton dashes to his office, where Clipper shoots and kills him for arranging the bets on Stanton from the other cities. The police then capture Clipper as Lee and Wellington arrive, having discovered the murder weapon, a water gun filled with poison. Back in his classroom, Moto explains why he suspected Benton and, as he goes to leave, discovers his timepiece missing. Wellington returns it, and Moto reveals that he now has Wellington's wallet, which, it turns out, Wellington stole from Lee.
