Imagine being able to walk through the streets of 15th-century Florence. Brunelleschi’s red dome soars above the Basilica di Santa Maria. Nearby, Giotto’s campanile shoots skyward, with Scripture and theology writ in stone along its entire length.

This is no mere model of a city: It is a populated, dynamic environment. Time changes from day to night. Merchants sweep the street in front of their shops, courtesans beckon from corners, cutpurses work the crowds, religious brothers and sisters walk the streets, people gather to watch street performers, and criers announce descriptions of the most wanted criminals. (Wait a minute: Is that you they’re describing?)

That’s what “Assassin’s Creed II” (Ubisoft, $60: Xbox 360, PlayStation 3 and PC) does, and does better than any game has ever done before. It places you in a glorious reproduction of a city at a certain point in time, then lets you explore.

Florence isn’t the only place given this deluxe treatment. A slice of the Tuscan countryside, a bit of Rome, and a marvelous reproduction of Venice are also yours to explore from any angle. Your character can climb up walls and leap from roof to roof, taking in panoramic views from, say, the campanile of St. Mark’s Basilica in Venice. The game is full of breathtaking moments like this.

It also contains some breathtaking stupidity. The “Assassin’s Creed” series is based on the utterly ridiculous premise that the memories of your ancestors are encoded in your DNA. Thanks to a machine called the Animus, created by a nefarious corporation called Abstergo, these memories can be stimulated and then simulated by a machine, thus allowing people to enter the lives of their own ancestors.

Abstergo is particularly interested in the genetic memories of a bartender named Desmond Miles, who’s descended from an ancient clan of assassins. In the first game, Miles experienced the memories of Altair ibn La-Ahad during the Third Crusade as he assassinated both Christian and Muslim leaders in an attempt to bring peace to the Holy Land. In a particularly risible bit of historical idiocy, the bad guys (both Christian and Muslim) are linked to a secret society called … the Knights Templar! (Abestergo is the modern face of the Templars.)

In the past and present, the Templars are looking for a relic called a Piece of Eden in order to bring peace to the Holy Land by robbing people of their free will. This relic is capable of creating illusions and is responsible for people believing in “supernatural” events such as the parting of the Red Sea and the role of the gods in Greek history. It’s a startling bit of antireligious nonsense, and it would be offensive if it didn’t seem to spring wholly formed from the imagination of a 15-year-old irritated about having to go to church. It’s simply too dumb to be offensive.

Also annoying is the portrayal of the Knights Templar as an evil secret society wreaking havoc throughout the centuries. Although exonerated by both the Church and historians, legends of Templar conspiracies remain common coin in popular culture. On the other hand, the Hashshashin (the real-life “assassins”) were a group of drug-addled murderers for hire who killed both Christians and Muslims.

Thus, it’s a little topsy-turvy for a history buff to find the Templars depicted as the power-mad villains and the assassins as the good guys. The fundamental problem with creating an assassin game is that assassins are evil, thus requiring some pretty serious truth-twisting to create a sympathetic character.

It’s too bad that the shadow of Dan Brown’s idiotic conspiracy thrillers lies so heavily on the storyline, since the gameplay is absolutely first-rate. The sequel picks up where the last game left off, but now Miles has to go back to 15th-century Italy and relive the memories of another ancestor. Ezio Auditore is a roguish Florentine lad who gradually finds himself caught up in a conspiracy after his family is wrongly executed for treason.

The plot is constructed around the historical Pazzi Conspiracy, an attempt to displace the Medici family by assassinating Lorenzo and Giuliano Medici at Mass. As Auditore gradually gains the skills and equipment of an assassin, he is drawn deeper into the conspiracy. He must avenge his family, stop the seizure of power by the enemies of the Medicis, and hunt down more Pieces of Eden.

He is no gleeful assassin. He attempts a kind of faux absolution before each victim dies, urging them to confess their sins and then uttering a requiescat. (All of the victims are villains, and the player is warned not to kill the innocent.) Gameplay is a combination of stealth, acrobatics and combat, with countless missions and subgames that will give it a long life beyond the main storyline. Like a webless Spiderman, Auditore is able to jump and climb over every surface in the city. His arsenal gradually grows to include retractable wrist blades, knives, swords, poison, smoke bombs, a rudimentary gun, and even a glider invented by Leonardo da Vinci.

The combat violence and adult elements earn it a “Mature” rating, but the blood can be turned off, and most of the swearing is done in Italian. There are certainly elements that will offend Catholics, including an absurd and anachronistic nun/prostitute, the attempted assassination of a pope (albeit the wicked Borgia Pope, Alexander VI), and the idea that religion is an illusion.

Balanced against this is a game that is not only fun to play, but which lovingly recreates Renaissance Florence and Venice in astounding detail. There are certainly problems of theology, morality and logic, but as a pure piece of game design, it works beautifully.

Thomas L. McDonald is editor-at-large of Games magazine

and a catechist in the Diocese of Trenton, New Jersey.