| Spoiler Alert!
Warning! This page contains spoilers for Underground.
|Publication Date||May, 2011|
|Genre(s)||Adventure, Science fiction, Young adult|
|Recurring Characters||Luke Hunter|
| Preceded by|
| Followed by|
Underground is the fourth book in The Phoenix Files and was first published May 2011.
Luke and Jordan are interrogated by the two people they confronted at the end of Mutation. They believe Luke, Peter and Jordan are working for the Cooperative and demand to know the location of Tobias, whom they have never heard of.
The interrogators' names are revealed to be Kara, whose mother established the complex they are living in and Soren, who is Kara's son. They are working to end Tabitha as Luke, Jordan and Peter are, but they still do not give each other their complete trust, despite that they deactivate their suppressors. Peter is being held by their custody as he is too dangerous to be near.
Kara and Soren are soon revealed to be Cat, Mike and Tank's overseers. They are angered by the reveal and Luke's dad finally arrives in Phoenix. The people of Phoenix have been growing suspicious and reports of citizens leaving the town have gone around, although they have actually become the latest victims of Tabitha. A new security camera network is set up in Phoenix making it harder for the group to enter the town.
Luke, Peter and Jordan tell their parents about Tabitha and reactions are mixed. Luke's mum does not believe him, whilst Jordan's and Peter's do. Jordan's mum and sister are arrested and her dad and Peter's parents join forces. Kara and Soren perform the same suppressor deactivation procedure on Peter's dad as they did with Luke, Jordan and Peter. After extensive research, a possible cause for Peter's condition has been discovered.
The dense forest surrounding Phoenix was not there 25 years before and the cause of the growth of the forest could be affecting Peter. The group go to the Medical Centre as the town is sent to a mandatory meeting in the Shackleton Building and rescue Jordan's mum and Georgia, Jordan's sister.
Much to Emily's dismay, Dr Montag is shot. The day after the rescue mission, it is revealed that the Shackleton Building has become a concentration camp for Phoenix's citizens and Peter's and Jordan's dads were caught. Luke and Jordan watch a video of themselves from decades ago where Peter stabs Luke. They are bemused and horrified to know that even if Tabitha is to be overthrown, Luke will die.