New York City wakes to a tense morning as Chief of Detectives McGowan [Robert Middlemass] receives a tip that the notorious international jewel thief known as the Lone Wolf is targeting the Bancroft pearls that very night. He assigns Detective Benson [Robert Emmet O'Connor] to the case, even though nobody knows who the Lone Wolf is; only former Inspector Crane [Thurston Hall] has had a clue about the elusive thief. The Lone Wolf, Melvyn Douglas as Michael Lanyard, has no trouble lifting the necklace, but his nerves flare when he spots policemen closing in on the mansion and he slips away by disguising himself in a butler’s robe and glasses.
Next door, at a masquerade, clumsy would-be thief Mal Mallison [Henry Mollison] almost steals an emerald pendant from Marcia Stewart [Gail Patrick], the glamorous hostess. Lanyard eavesdrops as Marcia ascends to store the pendant in a wall safe, and he learns that she herself longs for romance. After lifting the pendant, he borrows a mask from a drunken partygoer, slips downstairs, and dances with Marcia, planting a kiss that heightens the tension. Their moment is interrupted when the police arrive, searching for Marcia’s aunt Julie Stewart [Nana Bryant]; the party crowd unmasked as a precaution, but Marcia does not betray her guest. Liane Mallison [Tala Birell], Mal’s wife, recognizes Lanyard and quietly invites him to another party the next night, this time at Morphew’s, before he returns the pearls to the safe.
Frustrated with Benson’s progress, McGowan summons Crane, who immediately tracks down Lanyard. Lanyard reveals that the Lone Wolf has retired as of the previous night, a claim that leaves Jenkins [Raymond Walburn], Lanyard’s loyal valet, disappointed but loyal. Later, at Morphew’s gathering, Morphew [Douglass Dumbrille] offers Lanyard 25% of the proceeds from the coming robbery, a proposal Lanyard declines, and Morphew then informs Marcia that Lanyard is the Lone Wolf. The mood grows dangerous as Morphew’s henchmen gear up to take Lanyard away, but Jenkins engineers a prearranged police raid so Lanyard can slip through the trap.
When the lights go out, Lanyard finds Marcia and convinces her he has reformed because of her; she confesses she loves him too. As Lanyard exits Marcia’s mansion, the police shadow him, giving Morphew the perfect moment to steal Marcia’s jewels and plant Lanyard’s discarded cigarette at the scene. Crane arrives the next morning, prepared to arrest Lanyard, but Lanyard locks Crane in a closet to buy time. Lone Wolf calls Morphew’s men to warn that he’s loose, and the crooks panic, jumping into a taxi driven by Jenkins so they can rendezvous at Morphew’s place. Liane phones to check on the plan, while Lanyard, pretending to be Morphew, learns where the loot is hidden.
Jenkins drives the getaway, Morphew and the Mallisons converge at the suspected hideout, and Crane arrives with a search but finds no solid evidence. By the time Crane returns, Lanyard has already moved off with the jewels, leaving a note for the detective. A tense exchange of alibis unfolds as the phone line is left off the hook, and Crane quietly listens in as the criminals turn on each other. In the end, Crane brings them all to justice, while Lanyard returns the stolen jewels to Marcia and adds an engagement ring, sealing a fragile alliance built on romance and clever misdirection. The case closes with Marcia and Lanyard poised to begin a new chapter together, surrounded by the shadowy figures who swirled through Morphew’s party and the city’s relentless pursuit of a thief who could disappear in plain sight.
