Former "Dexter" stars Jennifer Carpenter (left) and Desmond Harrington (right), pictured here in episode 4 of "Limitless," reunited as a secret couple on the CBS movie spinoff. CBS

The plot continues to thicken on CBS’s “Limitless” TV series as Brian Finch (Jake McDorman) continues to juggle the complicated secret of his relationship with the superdrug NZT. In episode 5, the superhuman FBI asset starts to crack under the weight of all his secrets.

The episode opens with Brian sending a clever video message to his sober self explaining the complex situation in which he found himself at the end of last week’s episode. After stealing private files from his boss at the FBI to give to Ed Morra’s (Bradley Cooper) representative Mr. Sands (Colin Salmon), Brian has committed high treason. However, the files showed him information that his friend and FBI handler Rebecca Harris (Jennifer Carpenter) was right about her father being connected to NZT prior to his death. The problem is that if he tells Rebecca about this information, he’ll likely go to super-max prison.

So, Brian goes to work at the FBI the next day with the secret weighing heavy on him. Luckily, it’s not long before he’s kept busy with a sting operation to bust a local meth lab. Harris needs a code breaker to sit outside in the surveillance van and narrow down their search perimeter. He does it, but while the SWAT team is inside making the bust, Brian finds he’s drank too much juice and goes inside the building to use the restroom. Agents upstairs trip an explosive and, in the ensuing chaos, Brian finds himself face to face with a fleeing bad guy. Despite his NZT, it takes only one punch for the thug to take him out, but the police arrive just in time to arrest him.

This prompts Naz (Mary Elizabeth Mastrantonio) to demand that Brian receive self-defense training from agent Casey Rooks, played by Jennifer Carpenter’s former “Dexter” co-star Desmond Harrington. Similar to the actors’ relationship on the Showtime drama, they’re two cops that are secretly dating on “Limitless” as well.

Meanwhile, it turns out the bad guy’s name is Chris Garper (Derek Goh), and his brother Sam (Brady Dowad) has linked up with a hate group led by a man named Norris Allen (Ari Fliakos). The group, known as the Sons of Nathan Bedford Forrest, started out as an online hate-speech gang but has since evolved into a terrorist group that’s planning some kind of attack on New York City. Obviously, the FBI needs to interrogate Chris, but the only person he’ll talk to his Brian -- apparently punching him in the face gave them a special bond. The two get to talking and, thanks to his NZT, Brian is able to spend three hours with Chris learning everything there is to know about his brother. It turns out Sam had a secret e-mail address, which Brian and the department had little trouble accessing.

They discover that Sam was using the account to communicate with Allen about stealing something from a local hospital. It turns out that they’re planning some kind of radiation attack designed to disrupt New York City and the final component was located there. As Brian explains, the real problem with a radiation bomb is the panic that the city’s residence would create. Transit and business would shut down and, by the time decontamination is complete, the entire city’s economy will have collapsed. Their only chance is to get the information they need from Chris Garper, who has developed a bond with Brian. As a result, it’s left to Brian to reveal to the boy that his older brother has been confirmed as an aspiring mass murderer. Harris sees the emotional confrontation and commends Brian on his ability to do the right thing even if it feels wrong. This makes him consider telling her his big secret, but chooses to avoid prison instead.

However, his silence doesn’t mean that he can’t look into Rebecca’s father himself. He contacts Sands and asks if he and Morra would be willing to help him look into Conrad Harris. Unfortunately, Sands seems upset by the request and advises Brian to stop looking into it. He indirectly threatens Rebecca as he leaves, making it clear to him that his cooperation with them does not go both ways.

The bad news keeps on coming in the morning when Chris shows up at Brian’s door to tell him that Sam made contact with him. He wants to meet up so that Chris can deliver the last component he and Allen need to make their dirty bomb. Chris says he’ll help the FBI but only if Brian can get them to go easy on his brother when the time comes for sentencing. Brian pitches the idea to Naz, but she’s having none of it. After all, Sam set a bomb for her agents and is planning to destroy New York City. Giving him leniency wouldn’t exactly be being tough on crime and terrorism. So, the horrible task is on Brian to lie to Chris about his brother in order to get him to make the meeting happen. Thanks to his NZT, lying to Chris comes relatively easy.

What’s not easy is the meeting itself, which goes about as poorly as it could have possibly gone. Sam shows up with Allen, giving the FBI a chance to take down the whole terrorist plot in one fell swoop. Chris makes the handoff but gets scared and alerts Sam and Allen to the agents’ presence. He urges them not to run but they do. Chris and the agents give chase and easily nab Sam. As they’re putting cuffs on him, a vengeful Allen doubles back and shoots Chris. The FBI takes him out, but Chris has suffered a serious wound.

Later that night, after coming down from his NZT high, Rebecca informs Brian that, although he was integral in stopping the terrorist plot, Chris didn’t make it. Brian is beside himself with guilt, telling Rebecca that he doesn’t know how she can live in a world where you have to manipulate people and keep vital secrets from them in the name of a nebulous greater good. During his rant, he also reveals that he’d become wise to her not-so-secret relationship with Rooks.

Brian goes home and leaves one last video message to himself, this time from his sober self to his NZT self. He says that he’s not willing to become the type of person that the FBI, Morra and the world wants him to be in order to succeed in his mission. The episode ends with Brian knocking on Rebecca’s door and handing her a file that he says he refuses to tell her how he got.