When an introverted storyboard artist gets assigned to paint the picture of the culprit she has to infiltrate with the suspected underground network and paint all their portraits to find out who the killer really is.

This sounds like a great concept for a thriller.
I do wonder, who is "the" culprit? Maybe specify what the culprit has done. Same goes for "the" underground network, I'd replace that with "an" underground network. Also, a comma somewhere in the middle would be great. Something like this:
"When an introverted storyboard artist gets assigned a mysterious homicide, she has to infiltrate an underground network in order to unveil the killer's true identity."