'Chicago Fire' season 5 episode 12 spoilers: Firehouse 51 is under siege in 'An Agent of the Machine'

After dealing with the emotional fallout of last week's episode, this week's "Chicago Fire" has also set up a new storyline that puts the entire Firehouse 51 in danger.

When faced with a choice between saving a young girl or an armed man, Casey (Jesse Spencer) decided on the former, but in the process opened himself up to a vengeful plot that will plague Firehouse 51 in the episode titled "An Agent of the Machine." Although he did come back after making sure the young girl was okay, the man has already escaped through a secret hatch with a burning vendetta for the Chicago Fire Department.

His prime target may be Casey, the fireman who chose to leave him for dead, but his anger essentially puts everyone else in danger. So much so that the Chicago Police Department (CPD) is prompted to step in and help apprehend this man before he causes some real problems to Firehouse 51.

The trailer for the next episode have also hinted that Casey's life may be put in jeopardy as the man he left to die goes to great lengths to make him pay with his life. Can this man still be reasoned with, or has he already been completely consumed by anger that he'd willingly put everybody else's life at risk just to put Casey down?

Meanwhile, Dennis, an old buddy of Severide's (Taylor Kinney) father, comes to Firehouse 51 with the intention to shadow the squad. This comes after Anna (Charlotte Sullivan) left Severide and their almost-relationship in limbo. Will Dennis' arrival bring Severide the comfort he needs, or will the man's ulterior motives only add to Severide's emotional burdens?

Casey and Gabby (Monica Raymund) are only beginning to heal from having to give their son to the boy's real father. How will the events of the upcoming episode affect their progress? What is this present that the episode synopsis teases Casey will give to his wife? Will he give it to her before or after the attack on Firehouse 51?

"Chicago Fire" season 5 episode 12 airs on Tuesday, Feb. 7, at 10 p.m. EST on NBC.