John Fletcher as Oliver Hill
Alexis’ (Jessica Lu) former partner Oliver Hill (John Fletcher) warns Mara (Sarah Shahi) about the dangerous side effects of Reverie 2.0 in Season 1, episode 4 of NBC’s “Reverie.” NBC/Vivian Zink

Someone from Alexis’ past brings bad news to Mara in this week’s episode of “Reverie.”

According to the synopsis for Season 1, episode 4 of the NBC sci-fi drama, Alexis’ (Jessica Lu) former partner Oliver Hill (John Fletcher) resurfaces and warns Mara (Sarah Shahi) that Reverie 2.0 has some dangerous side effects.

In the promo for the hour, Oliver tells Mara that he knows what she’s going through. “How certain are you that you’re sitting in this diner right now?” Oliver asks the former hostage negotiator. “I know what’s happening to you because it’s been happening to me. That’s why I know you’re in danger.”

After learning that she isn’t the only one who’s experiencing side effects of using Reverie 2.0, Mara asks Charlie (Dennis Haysbert) if there’s something he’s not telling her. “There’s some kind of secret you’re trying to protect?” Mara asks the senior security consultant for Onira-Tech.

“Do you really believe that I would bring you here if I thought you could be hurt?” Charlie replies.

In order to clear Mara’s mind from everything Oliver has told her, Charlie confronts Alexis’ ex-partner. “Stay away from Mara. I won’t tell you again,” an intimidating Charlie tells Oliver.

As revealed by Paul (Sendhil Ramamurthy) in Season 1, episode 2, one of the side effects of using Reverie 2.0 is called derealization.

When asked to explain the said side effect, Paul told Mara that it’s a ghosting effect “like when you look at something bright and still see it when you close your eyes.” After Paul pointed out to Mara that Reverie 2.0 is programmed to recognize images she’s emotionally connected to, she admitted that she saw someone from her past (her late niece Brynn) while on the program.

After Mara shared that it has already happened twice but just for a moment, Paul said that it was just a temporary effect. Paul then advised Mara to put away anything associated with the person she saw on the program until the effect passes. He also gave her a bottle of mild anti-anxiety meds for her to take if something similar happens again.

Also in the next episode, a BCI is stolen at Onira-Tech and Mara must go into a rogue Reverie.

“Reverie” Season 1, episode 4, titled “Blue Is the Coldest Color, ” airs on Wednesday, June 20 at 10 p.m. EDT on NBC.