Vengeance

Polti considers Vengeance to be two different situations (3th and 4th) depending on whether the Avenger and Criminal are kinship or are not. Its Elements are

• Criminal
• Avenger

Plot: the Avenger punishes the Criminal

What was the crime? Murder, injury, insult, property damage; attempt of one of the previous.
Was the crime real, perceived, accidental?

Upon who was committed the crime? In Polti's 4th Dramatic Situation the victim was a relative of both Elements, although it is not needed because the need to vengeance is complicated just by the kinship relationship between the Elements.
Upon who is the punishment going to be committed? If the Criminal is not within reach, the Avenger may destroy its properties or take revenge upon the Criminal's family or friends or even everyone in a group the Criminal belongs to, like a deceived woman taking revenge upon every men because of the betrayal of one.

This situation is too short and simple to be a story by itself, it's more probably to be the cause of an story or the culmination of one.

Criminal

Is it the real culprit? 

PCs as Criminal

The PCs are punished in this story, so it is only a matter of seeing what the punishment is and what are the PCs going to react.

Avenger

Remember that the Avenger needs not to move because of personal reasons, but dutiful or professional ones.

PCs as Avenger

What are going to be the consequences of whatever the PCs decide to do to the Criminal?

Development

From the Avenger's point of view, this is often the end of a plot line, but it can develop into Erroneous Judgement if it realizes it was wrong. If then it does nothing to repair the damage, another investigator may appear and develop the story in a Remorse. Even if the Avenger don't realize the error, the new investigator poses the Enigma of whether the Criminal was really guilty.

From the Criminal's perspective, if it lives, this develops into Falling Prey of Cruelty/Misfortune.