Detective Justine Case is finally sitting down to write her memoir and wonders where to begin ("Sleuth in Duluth"). She takes us back many years ago to the home of Duluth theatre director Varthur McArthur, who's preparing for hosting murder mystery dinner party ("A Killer Party"). He explains that he has sent the guests invitations with information on what role they're playing and their costumes for the party. One by one, the guests arrive to the party. Once they are all seated for dinner, the lights suddenly go out. When the lights come back on, Varthur slumps over in his seat and goes facedown into his soup. At first, the guests think he is giving an authentic performance, but they quickly realize Varthur is actually dead, shot in the shoulder with a dart!

Justine Case, a meter maid, is at home when she receives a call. She is thrilled to finally be called on to work as a detective ("Today's Detective"). Tired of handing out tickets, Justine is ready for a new career. At the scene of the crime, Justine tells the party guests that they must self-isolate in separate rooms so she can question each of them separately. And much to their horror, she is confiscating their phones. One of the guests, George Murderer (it's a "family name"), thinks he recognizes Justine from somewhere. Alone in their rooms, the guests panic ("Stuck").

Justine interviews stage manger Clarke Staples (dressed as a stowaway) about what happened. Clarke gives a detailed recap of the night for her ("Boom! Shout! The Lights Went Out!"). Justine learns that Clarke went downstairs to the basement to turn the lights back on at the time of the murder. When asked about who has a motive for killing Varthur, Clarke explains almost everyone does due to Varthur's egotistical approach to running a theatre company. Clarke himself had hoped to finally direct a show during the season, only to be constantly let down by Varthur's false promises. Clarke then declares that he's certain that production designer Shea Crescendo killed Varthur. Justine finds Shea, who is still in character as Rosetta Stone (a performer on a circus steamboat). Justine learns that Shea screamed and ducked under the table when the lights went out. Shea admits that there was a fight with Varthur but tells Justine that he had another huge fight with new star Lily. Justine leaves, while Shea (as Rosetta Stone) sings for Varthur ("Circus On the Sea").

Justine finds chorus member Cameron Mitchelljohn in the kitchen, who aspires to be like Broadway and TV actor Jeremy Jordan. After Justine leaves, Cameron berates himself for stumbling over his words and looking so guilty. He coaches himself to calm down ("Breathe"). Lily Wright, alone in her room begins a ballad only to be interrupted by Justine ("Songus Interruptus"). Lily tells Justine that she has always talent, but diva Vivika always got the spotlight. Until recently, Lily was a licensed real estate agent. As Lily keeps going into song, Vivika interrupts. And then leaves. Lily concedes that Varthur gave her the leading role in exchange for doing a poor job selling the theatre. It turns out that Varthur's wife Joan hired Lily to sell the property. Because she hates Varthur's parties, Joan has been hiding upstairs the whole time. Before Justine leaves to find Joan, Lily tells her that George Murderer accosted her when the lights went out.

Justine finds Joan finishing an ASMR recording for a client. She has no idea what happened downstairs. When Joan is told that her husband is dead, she jumps for joy - and quickly clarifies that she did not kill Varthur, even though she really wanted to. Joan gives Justine her alibi - a Zoom call at the time of the murder. After Justine leaves, Joan excitedly faces the prospect of a new life without Varthur ("Live Out Loud").

While writing her memoir, Detective Case goes over the pivotal sixty seconds when the murder took place ("Boom! Shout! (Reprise)"). We then see Vivika alone in her isolation room. No longer the leading lady and trapped in the room, Vivika has no choice but to "Wait For My Cue". George sneaks into her room and they kiss. Vivika says there's something she needs to tell him, but then Justine knocks on the door. George hides in the closet. Justine corners Vivika, telling her she has plenty of motive and that everyone knows she's good at darts. Just as Vivika begins a confession, George bursts out of the closet, claiming he's the one who killed Varthur. They both insist they did it with a poisoned dart ("Never Miss My Mark"). However, Justine figures out that the poisons they used were useless and that the soup must have been poisoned - and Clarke brought the soup to the party. Justine rushes off to find Clarke, leaving George and Vivika alone together ("I Did It For You").

In the kitchen, Cameron is in a food coma when he gets a vision - Jeremy Jordan appears to him in a container of cream cheese! Jeremy gives Cameron a pep talk ("Be a Big Cat"). Cameron eats some soup when Justine bursts in and tells him to stop. Justine tests the soup for poisoning. Meanwhile, back at the crime scene, a mysterious set of hands begins to lift up Varthur. When the timer on Justine's poison test goes off, the hands put Varthur back down - but leave a handprint behind. The test shows the soup is not poisoned, leaving Justine back at square one. She heads to the scene of the crime and finds the handprint. She uses a paper towel to make a "copy" of the print. Justine goes to each suspect's room and to compare their hands with the handprint ("Hands Out"). But the only hand that matches it is Varthur's….

Still in character, Shea claims to know who committed the crime. At first, Justine brushes it off but then Shea shows pages from a script written by Varthur ("Shipload of Fools"). The guests act out the script. Thanks to the lyrics, Justine finds a box in Varthur's desk with a V-shaped stamp inside it. Justine spots what appears to be two photos of Varthur as a young boy and figures it out.

Justine gathers everyone in the dining room to reveal who did it ("Process of Elimination"). Justine is about to reveal something in the coat closet when Varthur starts clapping and sits up! However just as everyone starts applauding his outstanding performance, Justine reveals he is not Varthur. She opens the coat closet to reveal Varthur's dead body. It turns out the living "Varthur" is Arthur - Varthur's twin! Arthur reveals all ("A Killer Party (Reprise)"). As a child, Arthur played Gavroche in Les Misérables on Broadway, and one night, a jealous Varthur locked Arthur in his room and went onstage in his place. Varthur did so poorly, Arthur was fired from the show. Seeking revenge, Arthur had previously tried to shoot Varthur, only to miss and be imprisoned. Once he was out on parole, Varthur locked Arthur in his basement and made him ghostwrite his plays. Arthur's plan was to poison Varthur and switch bodies, allowing him to live a new life. Justine arrests Arthur, and we return to Detective Case writing her memoir ("Sleuth in Duluth (Reprise)"). The phone rings, and once again Detective Case is summoned to a new case.