Metropolitan monsters are a special class of mythic monsters. Metropolitan monsters can only be ranked up using their own cells, nebula or ambrosia. They also have the ability to perform a cap breakout, allowing them to rank up to intermediate steps with less Nebula. Additionally, players may have only one copy of each Metropolitan monster.
The Metropolitan Era Saga follows the story of Malfeitor and his son Malairion who, newly empowered from the discovery of a meteor of a powerful substance called Nebula which had hit Elektra's sun, attempt to keep it hidden from the populace of Monstown for themselves. They learn that the Nebula in the sun is causing the sunlight to have an energizing effect on the citizens, and so, in order to prevent them from benefiting from it, the pair of supervillains ally with Don Canine. Don Canine campaigns for mayor on the lie that the Nebula in the sunlight is harmful, and promises to convert the entire city into a massive building, called the Shelter, for the citizens to live in out of the sun's rays. In reality, the Shelter absorbs and concentrates Nebula from the sun for the villains to keep.
Meanwhile, Don Canine makes a deal with The BaroneX to secretly allow him to harvest iron ore and dump the garbage from the city's demolition in the Wastelands. In exchange he uses some Nebula to power up the BaroneX. After he goes back on his word and continues dumping on her land past the agreed limits, she regrets her actions and tells the Wastelands monsters what she had done. They elect Butcherang as their new leader, who leads them to raid Monstown and dump the trash back in its streets.
Ultimately, UV Fusion, spurred by her boyfriend VoltaiK's desire for acclaim leading him to corrupt himself and assist Don Canine in driving out the Wastelanders, confronts Malfeitor and Malairion in their base, defeating them and driving them off Elektra.