Dr. Andrea Segerstrom had stopped trying to control the nervous picking at her flaking, peeling nails. They'd always been soft and prone to splitting even when left alone. And in this situation her polished, tasteful French manicure had become a ragged mess. Nigel detested her ruined nails and the snags in his precious sweaters that followed them.
Then again, it was on Nigel's account that she was nervously ruining her manicure to begin with. I'd probably stop greying if I just divorced him already, she thought in anguish. Out loud she said, "But you don't know it's him."
"No we don't, ma'am," said the agent sympathetically. Andrea had already forgotten his name. He was a hulking giant of a man with a paradoxically cherubic face. "We don't know who's been compromised. That's why you're here, Doctor. We were hoping you could straighten this out."
"But I'm Nigel's wife. Aren't they concerned the emotional connection might affect my judgment?"
"Are you?"
"Every good psychiatrist recognizes they may be influenced by emotion and never even realize it."
"Dr. Segerstrom, you were the obvious choice. What with your work on SCP-1536…"
"I only directly researched subjects exposed to SCP-1536 and the long-term effects of SCP-1563 exposure. Yes, I used 1536 as a case study for Human-Impersonating Anomalies, but 1536 was hardly sophisticated."
The sweet-faced agent flipped through her file. "It says here your work on HIAs is seminal to the field. Who else could we call in?"
Andrea sighed before peeling off what was left of her right pinky nail. "Fine. Give me the relevant facts."
"At 0300 today, Area-29 experienced a massive containment breach due to an attack by the Chaos Insurgency. After all the other breached anomalies had been secured at 0430, it came to Area command's attention that SCP-378 was still at large. The entire Area has been placed under quarantine. Nobody comes in or out until we get this sorted out."
"So we have an entire Area's worth of personnel, one of which might have been compromised?"
"Not exactly. We were able to clear all but five personnel before our brain scanning equipment was compromised."
Andrea leaned forward. "Wait. If this site is under quarantine, why am I here?"
The agent sighed. "We don't have any medical personnel to spare. So if we get new scanners, they'll have to send all the technicians and doctors to operate it, and someone's going to have to get close enough to SCP-378's host to run the risk of being killed. And I'm sure you're aware that's how SCP-378 breached containment the first time. Or we could send the closest expert in detecting non-human anomalies and have you interview the five personnel being contained. They deemed the latter to be lower-risk."
Andrea leaned back in her seat. "Alright. Let me go over everything SCP-378 has ever communicated to us. After that, I'll speak to the five in quarantine."
The agent handed her a depressingly thin file folder. "Good luck," he said before leaving the debriefing room.
Agent Brenner. Agent Tom Brenner is his name, Andrea thought pointlessly before going to work.

Five different quarantine rooms. Five different windows, five different microphones. But only four humans in those five rooms. Can't hurt to start with the basics. Andrea couldn't help humming to herself as she interviewed the unlucky five.
Quarantined Subject One appears to be Dr. Erik Vanderhausen, Nordic male, forty years old. Blond hair, blue eyes, 2.1 meters tall. Senior Researcher, Microbiology, assigned to SCP-378. Married to Aimee Vanderhausen for fifteen years with two children, Sigrid, ten, and Adrian, two. He displays all the classic symptoms of shock. Visible pallor and sweat, and breaks into tears while reciting the names of his children.
"I just want to see my kids again. I can't be locked up forever, I have to tell my wife I'm sorry I fucked up the garden, she loves that thing, you know? I'm not SCP-378, please let me go!"
"Dr. Vanderhausen, I will ask you to remain calm. The Foundation hates false positives as much as it hates false negatives. If you are uncompromised you will return to your family unharmed."
One of these things is not like the other ones…
Quarantined Subject Two appears to be Tessa McAvoy, Scottish female, twenty-nine years old. Auburn hair, hazel eyes, 1.63 meters tall. Clerical Personnel, Clearance Level 1. Daughter of Sarah and John McAvoy and visits them frequently at their home in Arizona. Is notably calmer than the other four.
"Sorry Doc, you know these guys don't tell me anything. Men, huh? Between us girls, what's going on? So, the gas leak got fixed, right?"
"Sorry Ms. McAvoy, I can't comment on ongoing situations, but I can tell you I'm here to straighten things out. I'm just going to ask you some questions about your family."
"Don't you guys have that on file or something?"
"We just want to test your mental wellness in the wake of this event. Now if you could provide us with your date and location of birth, your immediate family members…"
"Fire away, Doc."
One of these things just doesn't belong…
Quarantined Subject Three appears to be Britney Howard, American female, twenty-eight years old. Light brown hair, light brown eyes, 1.51 meters tall. Clerical Personnel, Clearance Level 1. Is on friendly terms with Tessa McAvoy. Lives with her boyfriend of one year, Bradley Weston. Is nervous but not panicked.
"Look Dr. Segerstrom, I know you guys are working with some dangerous stuff and I'm kinda freaked out. It's not like I thought bioweapons research would be a walk in the park but I didn't sign up for this, you know?"
"I understand your concern. Your safety is our top priority."
"Did…did one of your samples get loose or something? I don't have cholera or some shit like that do I?"
"I can't comment on security breaches, but I am here to ask you some basic questions to gauge your emotional response to these events…"
Can you tell which thing is not like the others…
Quarantined Subject Four is Dr. Cheng-Gong Li, Chinese male, thirty-two years old. Black hair, dark brown eyes, 1.70 meters tall. Research Assistant to Dr. Vanderhausen. Engaged to Chan-Juan Wen, a biochemist at Area 19. Is belligerent and loudly insists on being released from quarantine.
"I don't understand! My name is Cheng-Gong Li, I was born in Hainan Province, I met my fiance Chan-Juan at Johns Hopkins and we're getting married three months from now. I'm not SCP-378, let me go!"
"If you're aware of SCP-378 then you know that it is a very sophisticated HIA and has access to the host's memories. I do not yet know whether or not you are really Dr. Li."
"This is ridiculous! Are you just going to keep us here until the host dies and the damn worm bursts out of the poor bastard's skull?"
"Please remain calm, Dr. Li."
"Don't you dare tell me to remain calm! I'm a Foundation researcher! I'll be dead by the time you incompetent fucks find 378! Let! Me! Out!"
By the time I finish this song?
Quarantine Subject Five is Dr. Nigel Segerstrom, Caucasian male, thirty-six years old. Medium brown hair, blue eyes, 1.83 meters tall. Clinical psychiatrist specializing in memetic hazards. Married to Dr. Andrea Segerstrom, Foundation psychiatrist specializing in the detection of human-mimicking anomalies. Is, like Ms. McAvoy, unusually calm and rational.
"Hi, Peach."
Andrea retained her professional demeanor while inwardly flinching at the pet name. "Hello, Dr. Segerstrom. As you're aware, there has been a containment breach of SCP-378. We would like to ask you some basic questions now."
Five's faint smile flickered out. "I'm sorry, Andrea. I really shouldn't have called you that. I suppose I was hoping the nickname would convince you it was me, but SCP-378 can't be rooted out with cheap gimmicks like that."
"Let's stick to the questions, Dr. Segerstrom."
"Of course, Doctor."
The rest of the interview was uneventful until QS-Five said plaintively, "I have an idea to catch 378."
Andrea shot him a guarded look. "Do you?"
Five sighed. "That's the thing. I don't know it'll be of any use to you, coming from me. You'll have to figure it out yourself. Not that I'm worried, of course." Five leaned back and smiled dreamily. "You always were the first one to find the best solution. I'll be out of here in no time."
Andrea suddenly felt that she would forget how to cry if she didn't do so soon. "Well, Doctor, that ends our chat. We will continue to interview you and the other quarantined personnel."
"Alright. I'll see you then."
As Andrea stood and turned to the exit, QS-Five called. "Andrea! Wait!" She turned.
Five was standing out of his chair, face pressed against the protective glass. "I was wrong. It won't work. I have no idea how to identify 378. It's all on you, Peach."
Andrea nodded perfunctorily. "Thank you, Dr. Segerstrom." She was very proud of herself for managing to keep the tears in until she was alone in her room.

Dr. Matthew Inglewood was not particularly thrilled to see Andrea. As Area 19's resident psychiatrist, he felt somewhat shafted by the decision to call her in. He was thus rather eager to prove himself. "Far be it from me to question your methods, but isn't it a bit unnecessary to apply Tier One PiDHIA? SCP-378 is a fairly sophisticated Human Impersonating Anomaly. It's passed Tier One since it was contained."
"The Protocols for the Detection of Human-Impersonating Anomalies are meant to be applied sequentially. And 378 is hardly infallible. Perhaps its access to host memories is imperfect."
"Is it?"
"Not that we can tell. But at least now we know that."
"So we apply Tiers Two through Four?"
"Did that."
Dr. Inglewood looked furious. "That's it then. The second we get a surgical machine down there, we open up their skulls. To hell with it."
"That might not be necessary." Andrea stood up, closing the yellowed paperback she was reading. "PiDHIA focuses on things the original person would know that the imitator would not. SCP-378 clearly has access to all the person's memories and personality traits. It's spoken about using the science of other planets to seed space. Its intelligence has given it quite a nasty little superiority complex."
"So?"
"I have an idea. Get me your resident mathematician," Andrea ordered as she tossed the book at Inglewood.
Inglewood seethed as he flipped over the paperback. It was a beat-up copy of I, Robot. He threw Asimov's beloved work in the corner of the room as he reflected this was a lose-lose situation for him. If Dr. Segerstrom failed, he was stuck with a bodysnatcher and if she succeeded, well, she succeeded.

"Dr. Vanderhausen, we have finished the first round of testing. This second round tests higher level logical reasoning. We are hoping that SCP-378 does not have access to this part of the human brain."
"Wait, what? I haven't been working on 378 long but that doesn't sound right."
"We have this protocol in place for a reason, Dr. Vanderhausen. You may take as long as you like. It's a simple test. Just do the best you can."
"Hah! No pressure, right? God, and to think I thought the A-levels were terrifying…"
Did you guess which thing was not like the others?
"Ms. McAvoy, we need to test if this recent emotional trauma has impaired your reasoning. If that is the case, we will make psychiatric care available to you. Please complete this test to the best of your ability. You have all the time you need."
"Uh, okay. I was really bad at math in high school though."
"That's fine. This test is like the LSAT in that it doesn't test prior knowledge, but reasoning ability. It shouldn't be difficult."
"Okay, Doc. For science, I guess."
Did you guess which just doesn't belong?
"Ms. Howard, we are concerned that this recent breach of security may have affected you negatively. Studies have shown that intellectual ability can be drastically harmed by emotional trauma. We're asking everyone to take this simple assessment of logical reasoning skills to gauge your mental health. It requires no prior knowledge and is not timed."
"Oh, okay. And when I'm done?"
"Slip it into this drawer and await further instructions."
"Whatever you say, Doctor."
If you guessed that this one is not like the others
"Dr. Li, this is the second round of our protocols to determine 378's host. We believe 378 exposure curtails high reasoning abilities at the expense of an increased recall of the host's memories and behavior. This assessment will gauge your math and logic abilities. Take all the time you need."
"The hell I will. This thing looks like the Rosetta Stone."
"Please don't be discouraged. This is a basic assessment and doesn't rely on prior knowledge."
"All right. I'll take it, and I'll show you. Not only will I prove I'm clean, I'll blow the damn thing out of the water."
Then you're absolutely…
"Dr. Segerstrom, in accordance with protocol, we are administering a basic test of logical reasoning skills. This assessment requires no prior knowledge and is not timed. It is our belief that SCP-378 may be able to access so much host information at the expense of higher brain functions. This assessment will help us make a decision."
"Oh, Andrea, you know I'm horrible with tests."
"Please complete the test to the best of your ability."
"Alright. Just promise me I won't be terminated for not having 378 as an excuse for failing."
Andrea paused. "I promise." She meant "I love you", and they both knew it.

The next time Inglewood encountered Andrea, it appeared his ill mood had spread to the wiry man with the mussed hair beside her. He was going through the stack of tests with a ruler, carefully scanning each line. "Ah! It's so obvious when it's written like that! Do you know how infuriating this is?"
"What?" asked Inglewood. He had a vague feeling that the frazzled looking gentleman was irritated by something completely different than the woman with them.
"Hello, Matthew! I believe we've solved your little problem here. I'm sure you know Dr. Steven Penderbrook. He's a brilliant man."
"I don't feel very brilliant right now, Dr. Segerstrom," Dr. Penderbrook mumbled, nose deep in his papers.
"And how, may I ask, did you accomplish that?" asked Inglewood with a remarkably tiny amount of resentment in his voice.
"Why, we already established that SCP-378 knows at least as much about humans as we know about humans. So how do we differentiate the normal humans from a cosmic parasite?"
"How?"
"By finding the something no human knows, but SCP-378 does." Dr. Penderbrook handed Andrea a sheaf of papers. "We know SCP-378 thinks very highly of itself and wouldn't pass up an opportunity to show off, especially when it thinks that passing as a human depends on it." She handed Dr. Inglewood ten pages of small, cramped handwriting. "Here's the question in pictoral form. You don't even have to know English to understand what you have to do. All you have to do is prove that every even number starting with 4 is the sum of two prime numbers."
"That's it?"
Dr. Penderbrook laughed bitterly. "That's it? Of course that's it! Except it's not it. That simple little statement is the Goldbach Conjecture. It's 260 years old and no one has proved it until now." Penderbrook waved the sheaf of papers in Inglewood's face. "Every mathematician in here's checked this out and it's perfect. There are lemmas in here no one's ever heard of."
"Excuse me?"
Dr. Penderbrook ran his fingers through his hair, somehow managing to make it look less unkempt. "Not only has this proof never been seen by a human, some of the mini-proofs that prove the proof have never been seen by a human. And yet all of us can tell it's sound," he said with the slow, careful air of someone being forced to explain something to an idiot.
Andrea chimed in gleefully. "Something no human on earth knows, but a correct answer known to be correct when we see it. Something that doesn't necessarily require foreknowledge, so 378 couldn't plumb its host's brain and know it doesn't know it."
"Well, our doctors are the best of the best. Maybe they felt the pressure and came up with it."
Dr. Penderbrook took on a frighteningly murderous expression for a man who seemed so mild. Andrea simply replied, "Well, the doctors are biologists and a medical doctor, the latter of which I assure you has a terminal terror of proofs. And that doesn't matter anyway," she continued as she delivered her coup de grace, "for I highly doubt that a clerical worker with a Bachelor's in Communications could pound out this masterpiece in five hours."
Inglewood sank into his chair. "So it's over then?"
Andrea smiled. "It's over. Now, if you'll excuse me, I need to talk to a psychiatrist. Know any good ones around here?"

Andrea was told it was a marvelous re-establishment of containment. They were planning to encase the body of Britney Howard in tungsten, but they had barely reached the sealing room when SCP-378 became wise to the plan. Five agents were severely wounded, but Agent Tom Brennan received a Foundation Star and a bionic leg for managing to encase the rampaging corpse in its metal chamber. Andrea was offered the chance to watch the mayhem, but declined, remembering the innocent-looking light brown eyes.
"I have to hand it to you, honey. Your plan was way better than mine," said Nigel.
"You know, you never did tell me what your plan was."
"Hell, I don't even remember it, that's how bad it was."
"You need to stop getting wrapped up in these situations. I can't always be there to save your sorry ass," teased Andrea.
"You know that was the plan all along, right? Trapped you into marrying me and now you're obligated to save me from bodysnatching hell worms."
Andrea kissed Nigel on the cheek. "Well, at least you've made one decent plan in your life."