Stardate 1224.3, Hetemit-9, site of an abandoned Illyrian colony where ion storms make investigation challenging.

Pike and Una note a storm approaching the shelter they’re in and order everyone to pack up and head to the rendezvous point. Pike has to go get Spock himself because the communicators are disrupted by the storm.

Pike calls out to Ensign Lance to leave his investigation and get ready to transport. Lance leaves a cupboard open with several glass vials and bottles sitting on the shelf.

Kyle has difficulty transporting the away team. Hemmer boosts the transporter with auxiliary power and they manage to get Una and most of the team on board.

Pike finds Spock who has found some previously undiscovered journals. The two gets to the rendezvous point but Kyle is unable to get a lock on them now that the storm is closer. Una instructs them to find shelter until the storm passes.

Una comes on the bridge and briefs them on Pike and Spock’s situation. Uhura reports that there is no communication possible during the storm.

Ortegas spots Ensign Lance stripping off his clothes in the corridor. She asks if he’s okay and he says that it’s too dark. He rubs his face on the frosted glass, saying he needs light on his skin. She continues past him but rushes back when he bashes his head through the glass. Ortegas calls for security.

Una is in her quarters and starts to lean in on her reading light. She directs the computer to increase the illumination in her room. She leans back, tears the collar of her uniform open. Her skin flushes dramatically. Once the flush fades, she contacts Dr. M’Benga to ask about the status of the landing party.

He reports several members of the landing party are in sickbay with strange symptoms. She reports to sickbay for an examination. M’Benga says he scanned the crew members who have an obsession with light for contagion but they appear clean except for a drop in Vitamin D in their systems.

Una checks in with Hemmer for possible issues with the biofilters. He gets to work running a high level diagnostic.

Una is able to let Pike know that some landing party members are falling ill but the connection is lost before she can provide details.

La’an reports to Una while she’s running a computer search on disease incidents in the Hetemit-9 colony. While they’re discussing the ethics of genetic modification, La’an starts to exhibit the symptoms of the illness.

Because La’an wasn’t in the landing party, they deduce the illness is spreading. M’Benga recommends full lockdown. Una immediately enacts the lockdown.

On the planet, Spock examines the journals he found while Pike paces.

Spock shares that this colony of Illyrians was trying to de-engineer their DNA in order to gain entry into the Federation because the Federation prohibits genetic modification in its members.

Looking out the window at the storm, Pike and Spock see fiery figures flying through the air.

Uhura wakes up in her sleep pod and finds her roommates running a simulation of a star for the illumination it provides.

M’Benga reports nearly fifty more cases. Chapel notes that Uhura’s unaffected even though her roommates are infected.

Hemmer arrives at sickbay and tells M’Benga that he must run a diagnostic on the emergency medical transporter. M’Benga protests that he has research in the transporter. Hemmer starts his diagnostic and the lights dim which is not what he expected.

M’Benga insists Hemmer leave. Once he’s gone, M’Benga turns the lights back on.

On the planet, the flame-like figures try to enter the library where Spock and Pike are sheltering. Spock suffers a laceration but Pike manages to shut the door on them. The entities continue to try to enter.

Una tries to figure out why Uhura didn’t contract the illness.

She figures out that Uhura didn’t get sick because she was in the dark when she was exposed. The contagion travels on light. Una suggests shutting off the lights. Chapel points out all the infected are essentially light-addicts. They decide to sedate everyone in sick bay and shut off the entire ship’s lights.

During the blackout, Una researches Illyrian genetic modification related to disease control. The computer alerts her to a blackout override in the transporter room.

She finds Hemmer in the transporter room trying to transport a piece of the planet’s mantle onto the ship.

As he rushes towards the mantle piece, she stuns him and reverses the transport.

Slinging him over her shoulder, she takes him to sickbay.

On the planet, the beings are close to breaking into the library. Spock theorizes they do not mean harm. The storm’s ionization peaks. Spock realizes it is reaching levels that are lethal to organic matter. They fall to the ground and the beings form a shield that protects them from the ionization. Once the storm passes, they leave.

Una arrives at sickbay. Chapel sedates Hemmer and M’Benga reveals his Vitamin D levels are dropping so he’ll have to be next. Una asks if her blood sample revealed something they could synthesize a cure from. She admits that she’s genetically modified. She’s Illyrian.

M’Benga says her genetically modified immune system burns out infections rather than creating antibodies.

In the sickbay, La’an sits up on her bed behind Chapel. While Una sedates M’Benga, La’an overpowers Chapel and deactivates the warp core containment field.

Una fights La’an while trying to reactivate the containment field. La’an accuses her of lying to her because she hid that she is Illyrian.

Una immune system activates again as the radiation levels reach toxic levels.

On the planet, the storm has cleared and Pike tries to hail the Enterprise to no success.

As they prepare to return to the rendezvous point in hopes the signal is stronger there, Spock voices his theory that the colonists chased the storms when they became sick with the compulsion for light. He hypothesizes they became the plasma beings that saved them during the storm.

As they discuss this, another record tube ejects from the library wall on its own. Spock identifies it as a record of the Illyrians’ efforts to renounce their genetic engineering to join the Federation.

At the rendezvous point, Spock notes that if they hadn’t reversed their genetic modification which probably included Una’s enhanced immunity, they wouldn’t have become sick with the light contagion and all died or became ion storm beings.

Una wakes M’Benga up and informs him that her immune system saved herself and La’an from radiation poisoning. Chapel was able to synthesize a cure from La’an’s blood’s antibodies.

Una seeks out La’an. They discuss Augments versus Illyrians.

Una reports to Pike. She offers to resign her commission and surrenders for judgment. Pike rejects her resignation. She reminds him that if he shelters her, they could come after him too. He welcomes that and tells her to let him worry about Starfleet.

Her next task is to fix the ship’s biofilter issue that allowed the contagion to get onboard.

Una checks in with M’Benga. She informs him that it was the emergency medical transporter that inhibited the biofilters when they beamed the landing team back. She shows him that Hemmer’s diagnostic found something being held in the pattern buffer. Because he was holding it, he didn’t let the space dock team upgrade his transporter.

M’Benga admits that he is keeping his terminally ill daughter in the buffer in hopes he can find a cure for her disease. He expects to be turned over to Starfleet for endangering the crew.

Una informs him that she’ll provide a dedicated power source from the warp core for his transporter to prevent this issue from happening again.

While Una records her personal log, M’Benga materializes his daughter so he can read her a story. He reminds her that even though they’ve read the story many times before, the ending could always change. Una admits in her recording to being an Illyrian. She then deletes it.

M’Benga’s story with his daughter revolves around an artifact with incredible power to either cleanse the land of evil or bring upon it a dark age.