Something Rotten! is a musical comedy about the first musical ever invented (this musical is a historical fiction, and not to be taken seriously). Nick Bottom is a failing writer trying to come up with a good idea. He goes to see Nostradamus, a soothsayer, who tells him the next big thing will be "A Musical!" After Nostradamus convinces Nick into doing one, Nick starts writing one about "The Black Death". Nostradamus convinces Nick to write one about omelettes after he sees a vision of the next big play (Hamlet) but misinterpreted it as Omelette.
Full Synopsis (Wikipedia) [Spoilers]
The musical opens with "The Minstrel" welcoming the audience to the European Renaissance ("Welcome to the Renaissance"). He tells the audience that "not everybody is getting what he wants", referring to Nick Bottom, who runs a theatre troupe with his brother Nigel. They are rehearsing for their upcoming play "Richard II", while William Shakespeare(referred to as "The Bard") is opening Romeo and Juliet. Lord Clapham, a patron who trusts the brothers and raises funds for their troupe, enters to announce that Shakespeare is doing "Richard II". The news outrages Nick, as Shakespeare has already done "Richard III", and the thought of going backwards seems absurd to him. He rants about his hatred of Shakespeare to the troupe members, who are horrified ("God, I Hate Shakespeare"). Lord Clapham leaves, telling the brothers he is stopping their funds unless they have another play by the next morning.
Nigel and Nick go home to their small house, and on the way Nick encounters Shylock the Jew. Shylock expresses a desire to help fund the troupe, but Nick rejects him as it is illegal to employ a Jew. Bea, Nick's wife, tells them the events of her day and how she acquired their dinner as she serves it. They are saving for a better life, and when Nick tries to open the Money Box, Bea smacks his hand away. Bea tells him how she could help them out, but Nick is ambivalent ("Right Hand Man"). Despite Nick's arguments, Bea goes out to do jobs that Nick claims are for men. As Nigel sleeps, Nick faces the real reason he hates Shakespeare: "The Bard" makes Nick feel self-conscious ("God, I Hate Shakespeare (Reprise)"). He wishes there was a way to top Shakespeare, and steals from the Money Box to see a soothsayer. He finds a soothsayer named Thomas Nostradamus (The nephew of the famous soothsayer Nostradamus). Nick asks him what the next big thing in theatre will be, and Nostradamus says that it will be "A Musical", a play where "an actor is saying his lines, and out of nowhere he just starts singing...." Nick thinks it is ridiculous but quickly warms up to the idea ("A Musical").
Later, Nick meets Nigel on the street. Nigel has just met Portia, a Puritan and the daughter of Brother Jeremiah, and they immediately fall in love. Nick tells him that he shouldn't pursue her because she is a Puritan. The Puritans leave and Nick tells Nigel what the soothsayer said, but neglects to tell him that it was not Nick's own idea. Nigel wants to do "The Brothers from Cornwall", the story of the two brothers' lives, but Nick vetoes saying it has to be bigger, and decides to do a play about the Black Death. The troupe performs a song for Lord Clapham, who is disgusted and deserts the troupe.
Nigel sits down to try to write a new play. Portia sneaks out to see him, and they discover more about their similarities ("I Love the Way"). A messenger arrives with an invitation for Nigel to attend "Shakespeare in the Park" and an after-party. Nigel explains to Portia that he sent one of his sonnets to The Bard for feedback. Nigel asks the messenger if Portia can be his "plus one".
In the park, Shakespeare performs for the people ("Will Power"). Nigel and Portia go to the after party, where Portia gets drunk. Shakespeare asks to read Nigel's journal of poems and writings, but Nick runs in with Shylock and chastises Shakespeare for trying to steal Nigel's ideas, as well reprimanding Nigel for his naiveté. Brother Jeremiah then runs in to find a drunk Portia and once again admonishes Nigel.
In a rage, Nick goes back to Nostradamus with what he has left of the money he stole from the Money Box. He asks Nostradamus to tell him what Shakespeare's new hit is going to be, and Nostradamus sees "Hamlet", but misinterprets it as "Omelette", among other mistakes (such as the Prince eating a danish pastry rather than being a Danish prince). He gets excited at the possibilities of success and dreams of a future in which crowds cheer for him and Shakespeare bows down to him ("Bottom's Gonna Be on Top").
The Minstrel welcomes the audience back and tells them of the stresses that the Bottom brothers and Shakespeare face ("Welcome to the Renaissance (Reprise)"). Shakespeare shows the stress he faces while trying to write hits and manage his fame ("Hard To Be The Bard"). A spy tells him that the brothers are trying to steal Shakespeare's upcoming hit. An excited Shakespeare decides to disguise himself as "Toby Belch" and audition for the brothers' troupe in order to steal the play.
Meanwhile, the troupe is rehearsing "Omelette: The Musical" ("It's Eggs!"). Shylock has become their new investor, though they cannot find a title that would make his role legal. When some of the actors become suspicious of Nostradamus and why he is at their theatre, Nick lies and says that Nostradamus is an actor. "Toby Belch" arrives at the theatre and is hired for the company. He is surprised to learn that his hit is about eggs.
Nigel sneaks out to London Bridge to see Portia, where he reads her another poem about his love for her. He worries about their future together, but Portia reassures him by saying that everyone, even Nick and Brother Jeremiah, will change their minds about their relationship when they hear Nigel's beautiful sonnets ("We See the Light"). Nigel is not very happy with "Omelette" and claims that it doesn't feel right. Brother Jeremiah interrupts the lovers and takes Portia away to be imprisoned in a tower for disobeying. Saddened by the loss of his love, Nigel becomes inspired to write a completely different play that is revealed to be Hamlet.
Nigel goes into the theatre the next day and tells Nick about his new improvements. They get into a huge argument and Shakespeare tries to take advantage of their squabble to get his hit ("To Thine Own Self"). A hurt Nigel scrambles out onto the street and is confronted by Shakespeare, who steals his hit under the guise of "improving it". Later, Nigel runs into Bea, who explains to him that they should still trust Nick because they can always fall on him if they need him ("Right Hand Man" (Reprise)).
Nick is having qualms about "Omelette: The Musical" as well, but dismisses these doubts once he learns that the town lined up all the way around the theatre for tickets. He and the troupe prepare for the show ("Something Rotten!"). Once the audience arrives, they perform a bombastic dance number that has many references to modern-day musicals (such as The Lion King and The Phantom of the Opera) ("Make an Omelette"). Towards the end of the number, Shakespeare reveals himself and sues the brothers. The troupe and Nigel find out that Nostradamus is a soothsayer, and are horrified.
In court, Shylock, Nick, Nigel, and Nostradamus are on trial and Nick is sentenced to beheading. Bea enters, disguised as a lawyer, and makes Nick confess that he stole from the Money Box, and tells the judge that beheading him would be redundant because he has already lost his head. She has made a deal with Shakespeare that they will be exiled to America ("To Thine Own Self" (Reprise)). She says that they always wanted a new country house and they are getting a house in a new country. Portia then arrives, having escaped the tower. She renounces her father's ideals and joins the Bottoms, Shylock, and Nostradamus in exile.
They arrive in America and tell the audience of the new opportunities in the New World ("Finale"). Nick hears about the opening of Shakespeare's new masterpiece, Hamlet, to which Nostradamus replies "I was this close".