Love Happy

Private detective Sam Grunion has spent eleven years chasing the valuable Royal Romanoff diamonds. His latest lead brings him to a troupe of struggling performers preparing the musical revue Love Happy. The pursuit threads through theater intrigue, theft, and a secret diamond racket.

Harpo watches Lefty unpack sardines in the Herbert & Herbert basement. He swipes a sardine can marked with a Maltese cross and swaps it with an unmarked one. The hidden can hints at a stash of diamonds; Madame Egelichi's suspicions grow and a reward is announced for Harpo's capture.

Lefty presents the wrong sardine can, enraging Madame Egelichi. She orders the police and puts out a $1,000 reward for Harpo's capture. The image of a diamond-smuggling scheme tightens around the theater and shop.

At the theater, Faustino the Great asks for a job as a mind-reader, and Mike hires him to keep the show afloat when backer money is short. Faustino's improvisation saves the production from repossession of the scenery. The troupe gains a new possibility with Faustino's tricks.

Harpo, secretly in love with Maggie Phillips, gives her the sardine can, and she promises she will eat them tomorrow. A policeman spots Harpo in the theater and takes him to Madame Egelichi, who interrogates him for three days while he remains silent.

Harpo escapes Madame Egelichi's suite and finds the diamonds inside the sardine can that had been left out for a cat. He pockets the diamonds, realizing their true value. This discovery sets up the night’s later misdirections.

Harpo takes Maggie to Central Park, where he plays the harp for her and gives her the diamonds as a birthday gift. The performers observe a growing romance, while the diamonds' whereabouts remain a secret. Maggie faces the risk of discovery as the plan unfolds.

On opening night, a Romanoff family agent visits Grunion and threatens to kill him unless he produces the diamonds within an hour. The pressure heightens the tension as the show goes on. Grunion hides, hoping the diamonds will surface through the evening.

Lefty and the Zoto brothers spy through a window as Maggie wears a diamond necklace; Mike pleads with her not to wear it and promises an engagement ring instead. Maggie removes the necklace and drops it onto the piano strings as the performance continues.

Faustino plays the piano with force, causing the diamond necklace to fly into the air and drawing Madame Egelichi's attention in the audience. Faustino pockets the diamonds and rushes to the roof to help Harpo.

Madame Egelichi arrives with a gun and demands the necklace; Faustino hands her fake diamonds while the real diamonds are recovered after tying up Lefty and the Zotos. Harpo then encounters Grunion on the roof, drops the diamonds into Grunion's pocket, and steals them back as Madame Egelichi leads the detective away.

Back in his office, Grunion remarks that Harpo disappeared with the diamonds, never realizing their true value. He takes a phone call from his wife, who turns out to be the former Madame Egelichi, revealing the final twist.