Scarpia is a bad guy, Cavaradossi is a good guy, Angelotti broke out of jail and Cavaradossi hides him. At first Cavaradossi's side loses, then his side wins, but he's doomed anyway. The end.
Then again, you may find yourself a little curious as to what was so important that torture, suicide, execution and murder resulted. The libretto, based on Victorien Sardou's 1887 play, refers to historical events without explaining much about them; it's assumed you already know.
So, if you don't know, here's a short summary of what all the fuss was about.
The French Revolution ended in 1799 with the execution of Louis XVI, replacing the monarchy with a military government led by Napoleon Bonaparte. Napoleon fully intended that revolution would not stop there; he was determined to march into the Italian peninsula and bring the ideals of liberté, égalité and fraternité to Rome, replacing the Papal states with a republican government.
Opposing Napoleon was Queen Maria Carolina, an Austrian noble who married King Ferdinand IV of the kingdom of Naples. Ferdinand was a useless good-time Charlie, uninterested in affairs of state. Maria Carolina, on the other hand was shrewd, ambitious and scared.
Scared of what? Well, her sister happened to be an unlucky royal named Marie Antoinette.
Marie's fate prompted Maria Carolina, who initially favored an enlightened approach to governing, to make an about-face, violently opposing any hint of revolution in Rome. The actions she took included utilizing the Austrian army to oppose French forces at the French-Italian border, and hiring thugs from Sicily to come up to Rome as hired goons, giving them license to stamp out Roman revolutionaries by any means necessary. (Scarpia represents that aspect of her efforts.)
She also sought the support of the Roman Catholic Church, traveling to Rome for pow-wows with church leaders. The Church recognized Napoleon as an existential threat to its power, making them eager to ally themselves with her plans.
One of Napoleon's generals, Gen. Berthier, did set up a Roman Republic as early as 1798, but it wasn't popular with the citizens and the Queen's resources caused it to fall in 1799. One of it's leaders had been a certain Signor Angelucci, who becomes Angelotti in the opera.
But Napoleon was not to be deterred by this first setback. He began marching towards Italy (which was not yet the unified Italian nation we know today, but a collection of Austrian territories and city-kingdoms).
He was opposed by his Austrian counterpart, General Michael Melas. They met near the town of Marengo, in the Piedmont region right at the French border. This battle was waged on June 14, 1800. The Battle of Marengo is the galvanizing event that drives all the action in Tosca.
The opera begins on June 17, just as reports from the battlefield are trickling in. Without wire services or cable news, news of the battle trickled into Rome via couriers on horseback. If you know the opera story, you'll recall that the Sacristan enters in the middle of Act 1, agog with excitement. He's just heard the result he was hoping for: that scoundrel Napoleon was sent packing! Woo-hoo!
This was in fact good news for most Romans, who were content to live their lives in the warm (if suffocating) embrace of the Church. The majority would have regarded men like Angelotti as terrorists. This is the impetus for the final scene of the act, a spectacular service of thanksgiving to God for delivering the city from the evil French army. This is the Te Deum that brings down the curtain.
(Reality check: this Te Deum could not have been organized as quickly as the opera depicts. There really was a grand Te Deum when that Roman Republic fell in 1799; St. Peter's square was lined with 4,000 soldiers!. That takes some time to arrange logistics. But never mind, it's a great scene.)
At this point, I will point out a more recent event of history that is similar to the confusion of the Battle of Marengo: the American presidential election of 1948. Early returns favoring New York governor Thomas E. Dewey led the Chicago Tribune to publish an edition with the banner headline: DEWEY DEFEATS TRUMAN.
As it happens, Harry S. Truman had the last laugh, as the final vote count landed him in the White House. The photograph (shown above) of Truman gleefully displaying an iconic example of #FakeNews is a reminder of the adage about counting unhatched chickens.
Melas's apparent victory is the great #FakeNews of Tosca. Napoleon rallied his troops at the last stages of the battle, pulling out a dramatic victory. Puccini and his librettists, fully recognizing the dramatic potential, used Sciarrone's announcement that "Melas has fled" to bring about a moment of potent theatrical effectiveness: Cavaradossi, weakened after having been tortured to try to force him to disclose Angelotti's hiding place, staggers to the horrified Scarpia for a moment of "IN YOUR FACE". (That's my updated loose translation of the Italian "Vittoria! Vittoria")
The upset win by Napoleon had the effect of a bomb going off, both in Rome and in Scarpia's apartment at the Farnesi Palace. It literally brings about the doom of all three principal characters. Scarpia's rage at Cavaradossi's gloating leads to the order to execute him. That in turn leads to Tosca and Scarpia's negotiation for her lover's freedom, resulting in the police chief's murder and the diva's suicide.
The Battle of Marengo instigated a half-century or so of instability in Rome and the rest of the Italian region that waxed and waned until unification was finally achieved in 1870.
Thus, Tosca was written as a centennial observance of the events of a century earlier. As is often the case, the point of view of the opera's events has more to do with changing perceptions of a later time than how those events were seen as they were happening.
For Puccini, the alliance of Church and State was an entity of oppression, tyranny and evil. As noted above, however, Roman citizens had a far different view.
By the way, another aspect of the Battle of Marengo is Chicken Marengo, a dish that has come down to us today. French supplies were running low, so Napoleon had his troops forage for food in neighboring villages. They returned with the ingredients for a recipe calling for chicken, garlic, tomato, eggs and crayfish. Want to taste history? Here's a recipe from the New York Times that substitutes mushrooms for crayfish. It must be authentically French - the author's name is Pierre. Can't get more French than that...