Much of this will be familiar from D&D – the big difference is how damage is handled.
At the start of the round, roll D20 + Initiative Rank. Base Initiative Rank is Agility.
Surprised characters are Vulnerable (1/2 active defenses), surprise round is without Move action.
A round is a Standard Action, a Move Action, and Free Actions.
Each Attack type you are capable of basically comes down to just one number – the number added to your D20 roll.
For instance an Unarmed attack Rank would be your Fight Rank plus Close Combat: Unarmed Rank.
Your Throwing Attack would be Dexterity + Ranged Combat: Throwing.
An attack is made against the targets Defense – which on is dependent on the nature of the attack.
For physical attacks it will be Dodge for ranged attacks or Parry for Close Combat (melee) attacks.
A critical hit (Natural 20) can give you a +5 bonus to damage roll, or an added/alternate effect of your choice at Rank 0. Like Blinded, Dazzled, whatever you can justify.
Each attack has a Damage Rank.
For Unarmed Combat it would the Strength, for example.
The defender makes a Toughness check against your Damage Rank + 15.
If you fail by -1 to -4 (one degree of failure), you have a -1 penalty to all further checks against damage.
If you fail by -5 to -9 (two degrees of failure), you have a -1 and are Dazed for one turn (can only take one standard action)
If you fail by -10 to -14 (three degrees of failure) you have a -1 and are Staggered (can only take one standard action and -1 speed).
If you fail by -15 to -19 (four degrees of failure) you are incapacitated and out of combat.
In general combat in M&M is not lethal. At the GMs discretion a hit that incapacitates may kill the character instead (this will be announced ahead of time). Additionally a nasty villain may kill an incapacitated character. I may rule that a hit that has more than four degrees of failure kills a character outright.
You remove one damage condition per minute of recovery. So first Incapacitated, then Staggered, then Dazed (if permanent), then one minute for each -1 penalty you have acquired. The GM may rule that lethal damage takes longer to Recover.
So in the end things are actually kind of simple.
For every Attack, a character has two numbers – the Attack Rank and the Damage Rank.
The defender has two numbers, his Defense Rank and his Toughness Rank.
If the Attacker’s roll beats the defenders Defense, then the defenders Toughness roll must beat the attackers Damage or he takes a hit.
During combat we keep track of your penalties instead of hit points.
Eventually, even a super-tough guy will take a hit that incapacitates them.
Note that modified rules exist for campaigns that want a more lethal experience.
Note also that I may end up simplifying things in practice….