Diana Skouris (Jacqueline McKenzie) is a scientist who worked with the CDC for seven years prior to joining NTAC, where she is now a lead investigator assigned to The 4400. Partnered with Tom Baldwin, the two were instrumental in exposing the government's secret promicin inhibitor program thereby saving thousands of lives.
Although initially wary of the 4400, Diana quickly formed a bond with abductee Maia Rutledge. Their connection grew stronger, and Diana eventually adopted Maia. Now, as she adjusts to her new life as a parent, Diana struggles to remain objective in her work at NTAC while raising a 4400 of her own.
She began a romantic relationship with NTAC analyst Marco Pacella, but ended it after an experience with a 4400 ability convinced her they had no real future together.
Diana continues to have a fraught relationship with her sister, April. The two have extremely different personalities and find themselves constantly at odds with one another. When April learned of Maia's precognitive ability, she took advantage and attempted to improve her financial situation. This only served to further strain the relationship between the two sisters. April soon resurfaced with a new boyfriend, Ben Saunders, but the relationship was immediately complicated by Maia's prediction that Diana would be the one to ultimately marry Ben. When Ben broke it off with April, his chemistry with Diana quickly turned into a full-blown romance.
When Maia's school was attacked and her daughter nearly killed, Diana decided to turn in her NTAC badge. Seeking to protect her daughter, she moved to Spain with Maia and new boyfriend Ben to start a new life.
Diana and Tom try to find the source of a computer virus that is disrupting ATM's and causing computer failures all over the country. With the help of Marco, they trace the disruptions to Warren Trask, a man who was swindled by Drew Imroth, the head of Ubient. Even though on life support, Trask has used his ability to go into the internet and disrupt it from inside.
When Trask is killed, Diana and Marco find evidence implicating Tom as the possible murderer. Diana and Meghan realize that Tom is marked and that someone else is inside him. They decide to try to act like nothing has changed until they find a solution to get the real Tom back.
Tom begins to have memories that are not his, and he realizes that someone or something is inside him. One of his disturbing visions is of himself surgically impregnating Lily Tyler in the future.
Rebecca Parrish, the director of National Intelligence, visits NTAC and tells Tom and Diana that she is not happy with the way things are going in Seattle. She orders them to find Richard Tyler and bring him back to NTAC.
Later, Parrish confronts Tom and tells him that if he feels like someone is inside him, he should stop fighting it and just relax. The next morning, Tom finally gives in and he is now one of the Marked.
Parrish meets with the "new" Tom and warns him not to blow his cover. She suggests that he do what the old Tom would do: find Richard and Isabelle Tyler. Tom does just this, when he and Diana go to a house and find Richard, Isabelle, and Cora hiding out. Tom shoots Richard and Cora with a tranquilizer gun and secretly takes Isabelle back to his house.Richard and Cora with a tranquilizer gun and secretly takes Isabelle back to his house.
Although Parrish is happy with their success, Diana begins to get suspicious about this "new" Tom.
Diana awakes to find herself trapped at NTAC with many of her co-workers as well as Collier, Kyle, and Isabelle. Eventually, PJ confesses that they are all trapped because he took promicin and developed the ability to draw others into a game that takes place in a shared psychic space. In the game, the players have to work together to overcome an obstacle; in this case, the obstacle is NTAC itself. When the building kills off Meghan, Shawn, and PJ, Collier and Tom join forces, however briefly, and save everyone by short-circuiting the building.
During the game, Maia asks Diana why she and Marco no longer hang out. As a result, Diana has a heart-to-heart with Marco and invites him over for dinner.
Marco informs Tom and Diana that a 4400 named Curtis Peck has disappeared. Peck's ability is to use his filmmaking skills—or lack thereof—to reveal the truth about conspiracies. Peck's latest conspiracy involves the Marked, people who've come back from the future and implanted their consciousness into ten prominent people in order to stop Jordan Collier.
After tracking down Peck, Tom and Diana discover that Drew Imroth, the CEO of Ubient software, is one of the marked. When Tom and Diana confront Imroth, he denies any knowledge of this strange theory, even though inspection reveals he does have a mark behind his ear.
Soon after, federal forces take Tom to a psychiatric evaluation center. While Tom is gone, Diana finds Curtis Peck and sets him up to write at Marco's place. Before Marco can finish his script about the Marked, he suddenly aborts it.
Later, Tom calls Diana with disturbing news. While he was sleeping, they injected him with something and he might not be himself anymore.
Before leaving for Evanston, Tom tells Diana to cover for him while he's away. Meanwhile, Diana tells Marco about Maia's recurring nightmare in which p-positives are being rounded up and taken into custody with malicious intentions.
Later, Diana tells Shawn about Maia's nightmare, and that in this vision, Gabriel Hewitt was some type of leader. She tells Shawn that by opposing Hewitt he may be inadvertently giving a platform to a very dangerous man.
Diana finally tracks down her sister April, who now has the ability to make people answer her questions truthfully. Diana is alarmed to find out that April has been using her ability to blackmail people. Things get worse when April gets herself in the middle of a dangerous corporate conspiracy. By utilizing NTAC's resources and April's powerful ability, Tom and Diana are able to get a confession from the conspiracy's ringleader. However, because April took promicin – which is now illegal – she worries that NTAC will now put her in jail. Eventually, NTAC decides that April's ability is too useful to waste on a prisoner and they hire April to work for them. Diana is overjoyed and relieved.
Meanwhile, Ben wants to know when they will return to Spain. After some thought, Diana tells him that she belongs in Seattle at NTAC with Tom, and that they can't go back.
Meghan tells Tom and Diana to find Audrey Parker, an elderly woman who is blogging about how promicin changed her life. When Audrey quotes Benjamin Franklin in her blog, Tom and Diana realize it is a woman they’d previously met at the library. But by the time they go to see her, she is dead. However, Audrey's astral projection is able to communicate through radio static. She uses this method to tell Tom and Diana that her nurse's son was the one who killed her.
Diana comes home one night to find Maia and Ben waiting for her. The lease was up in San Sebastian so they decided to move back to Seattle.
Diana, now without the power and privileges that come with NTAC, is having trouble locating her sister April. After consulting with Tom, Diana decides to seek reinstatement with Meghan Doyle at NTAC. Although Meghan knows that Diana has ulterior motives, she reinstates her anyway. When Garrity is hospitalized after his interaction with Brandon, a p-positive autistic boy, Diana and Tom partner up to track down the boy and his mother. Tom and Diana sedate Brandon and bring him back to NTAC. Shawn is able to heal the boy, ridding him of his autism.
After spending three months in Spain with Maia and Ben, Diana receives a jobn offer with the European union. Later, Marco calls Diana and tells her that April may have taken a Promicin shot.
SEASONS ONE-THREE TIMELINE
Dennis Ryland pairs Diana with Tom Baldwin, an NTAC agent who recently returned to duty after a family trauma. The two agents lead the investigation into The 4400.
Diana, initially wary of The 4400, softens after meeting Maia Rutledge, an 8-year-old returnee with the ability to see the future. Shortly after, Diana officially adopts Maia and the two attempt to create a normal family life together.
Their happiness is short lived as a disease starts to plague not only Maia, but other 4400s as well. With Maia placed in quarantine, Diana struggles to find a cure to save her daughter.
As Tom and Diana investigate, they discover that NTAC is, in fact, behind The 4400 illness -- the organization has been injecting returnees with a promicin inhibitor that regulates their abilities. Diana correctly speculates that Ryland is responsible.
Using a serum made from Isabelle's blood, Diana is able to cure Shawn Farrell. With his unique healing ability, Shawn is able, in turn, to heal his fellow 4400s, including a deathly-ill Maia.
As their lives get back to normal, Diana begins to spend more time with fellow NTAC employee Marco.
Following Ryland's shooting, Tom and Diana are assigned to track down TJ Kim and protect Ryland's co-conspirators.
NTAC receives a message from the NOVA group, a radical wing of The 4400, who are threatening to take action on October 19th. Tom and Diana attempt to investigate.
Using 4400 Gary Navarro, Tom and Diana begin to hunt down the members of the NOVA group. Unbeknownst to them, Gary is leading Tom and Diana in the wrong direction.
Diana surmises that Gary has been lying to them the entire time and they successfully arrest him before he can kill Ryland. During Gary's interrogation Tom and Diana learn he's working for Daniel Armand at NOVA.
With the help of Shawn, Tom and Diana arrest NOVA member TJ Kim and prepare for the events of October 19th. But what they expect to be a disaster instead turns into a miracle. The NOVA group is clearly sending a message – they can use their powers for good or for evil.
After visiting Dr. Kevin Burkhoff, Diana learns he has been injecting himself with a promicin serum. She agrees to help Burkhoff through his experiments.
Diana decides to send Maia to The 4400 Center school.
After Maia disappears with a woman claiming to be Sara Rutledge, Diana goes into a panic trying to find her daughter.
Plagued by visions of a young girl, Diana discovers that the future abducted her daughter and erased her from everyone's memory. With the help of Tom and Alana, Diana is able to visit Maia and ultimately bring her back to the present.
Diana and Tom track down Daniel Armand only to find out that Isabelle has gotten to him first. After seeing what Isabelle can do, Diana is convinced that she will kill again before she is stopped.
After tracking down most of the NOVA group, Diana and Tom attempt to arrest Gary Navarro only to find he's fled his house. Concerned that someone tipped him off, Diana and Tom investigate and learn Dennis Ryland has also been pursuing the NOVA group. Although Diana and Tom try to exchange Gary for Alana (who is in custody for warning Navarro), Diana can't handle the guilt and eventually they release them both to live a life on the run.
After eating cookies laced with Blink, a new hallucinogenic street drug, Diana has visions of an ex-boyfriend Josh. Her hallucinations make her realize that she has unresolved issues and Diana decides that that she's not being fair to Marco. She ends their relationship.
After finding Kevin Burkhoff hiding out with Tess Doerner, Diana begins to feel ill. After she consults a doctor, Diana realizes that she (under Tess' control) has been injecting herself with promicin.
Diana and Tom discoverer the Starzl Mutation, a rare abnormality seen only in 123 people in the Seattle area. It turns out that the only way 4400's can have children with abilities is to reproduce with someone with the Starzl Mutation.
Diana and Tom learn that Haspel Corp has been injecting participants in an enhanced soldier program with promicin to create an army with 4400 abilities.
Diana finds that working with Marco is still awkward.
Diana meets April’s new boyfriend Ben. Soon after, Maia tells Diana that she had a vision that Diana will marry Ben. Although Diana tries to deny it, she and Ben can't hide their attraction for one another and begin sneaking around behind Aprils back.
Maia tells April about Ben and Diana’s blossoming relationship. Under the weight of their guilt, Diana and Ben agree to spend some time apart.
After NOVA members manage to escape from NTAC, Diana and Tom attempt to avoid a confrontation between Collier/the 4400 and Ryland/his enhanced army. Along the way, Diana learns that Isabelle has been working with Ryland but more astonishingly, that Collier managed to steal Ryland's entire supply of promicin.
After learning that someone plans on distributing promicin publicly, Diana and Tom suspect Collier is behind it all. When they discover Shawn trying to heal a dying Devon, he is brought into NTAC and although Diana and Tom try to get information out of Shawn, they are unsuccessful. Worried The 4400 Center is no longer safe, Diana pulls Maia out of school. While home schooling her daughter, Ben arrives and asks Diana to take Maia and move away with him. While Diana contemplates Ben's offer, she and Tom track down Isabelle at The 4400 Center, engaging in a power struggle that eventually leaves Isabelle without her abilities. With the problem of Collier and the 4400 momentarily behind him Diana and Maia decide to leave for Spain with Ben.