When things start happening quickly, characters use their initiative bonuses to determine who goes first. Each character involved in a conflict makes a check of d20 + initiative modifier,.
Characters then act in initiative order, from highest to lowest.
The order in which characters take their turns is determined by initiative. Base initiative bonus is equal to the character’s Agility rank. Many characters have advantages or powers that modify their initiative, such as Improved Initiative. At the start of a conflict, roll an initiative check for each character:
The initiative check determines what order characters act in, counting down from highest check result to lowest. Usually, the GM writes the names of the characters down in initiative order to move quickly from one character to the next each round. You can also have all of the characters’ names listed on index cards you can reshuffle to fit the initiative order. If two characters have the same initiative result, they act in order of highest Dodge bonus first, then highest Agility and highest Awareness. If there is still a tie, each tied player should roll a die, with the highest roll going first. The GM may roll just once for an entire group of minions, giving them all the same initiative.
If characters enter a conflict after it’s begun, they roll initiative when they join-in and act when their turn comes up in the existing order.
Mutants & Masterminds Hero’s Handbook, Copyright 2011, Green Ronin Publishing; Author Steve Kenson. Chapter-3 Abilities, Pg 54-59