Do NCIS přijde dopis s otiskem rtů na obálce. Právě DiNozzo ho díky své povaze otevře. Z dopisu se vysype bílý prášek. Agenti z doprovodného textu zjistí, že se skutečně jednalo o biologický útok. Gibbsovi lidé jsou v izolaci a lékaři s Abby hledají pachatele a látku, která byla v dopise obsažena. Zjistí, že se jednalo o spóry plicního moru.

Další vyšetření ukáže, že nakažený je pouze DiNozzo. Mezitím se objeví první stopy po pachateli. Tím je matka údajně znásilněné dívky. Dívka tvrdila, že ji znásilnili nějací námořníci. Matka je ředitelkou laboratoře, která právě pracuje s nebezpečnými viry a bakteriemi. Gibbs však dokáže, že údajné znásilnění si dívka vymyslela. Naštěstí i DiNozzo se začíná uzdravovat.

Kate and Tony enter the squad room. Tony asks Kate if she spent the night Saki Bombing. Kate doesn't know what Saki bombing is and states that she has a cold. Tony then tells McGee that Kate didn't know what Saki Bombing was and McGee replies that he didn't know either. Tony remarks that he works with a bunch of "wankers," to which Gibbs replies as he walks in, "and you make three, DiNozzo." Kate sneezes and Gibbs stops, walks back to Kate's desk and asks her if she is sick. Kate tells him that she has a cold but she will sneeze into her tissue, unlike some people (while she looks at Tony). Gibbs tells them that he has never had a cold or the flu. Kate is surprised and Tony remarks to her "If you were a bug, would you attack Gibbs?" to which she laughs.

McGee is passing out the mail and comes across a letter addressed simply to NCIS special agent. Tony notices the SWAK on the back and snatches it from his hand, saying that it's for him because he "recognizes the lips". Tony opens the envelope and, with a slight smile on his lips, blows into the envelope. A fine white powder flies out and envelopes his head.

Gibbs stands up on his desk and whistles to get everyones attention. He announces that an envelope was opened and an unknown substance was released. He then tells everyone to evacuate. The squad room goes into emergency mode and Kate tosses Tony a bottle of water as she calls security to tell them what is happening. Kate hears the air conditioning turn off, then says to McGee and Tony, "let's hit the showers" to which Tony replies, "I thought you would never ask." As Tony passes Gibbs, he apologizes. Gibbs looks around the now empty squadroom.

Our team has to go into decontaminating showers, while the squad room area is quarantined. While showering, Tony, Kate & McGee discuss who would send Tony a letter with anthrax. McGee states it may not be anthrax, it could be small pox, bubonic plague, cholera, foot powder, face powder, or talcum powder. Tony comes up with "Honey Dust" he gives it to girls as a gift at Christmas. Kate and McGee have never heard of Honey Dust, but Gibbs comments that it makes a woman's skin feel silky smooth, and when kissed it tastes like honey. He got a box last Christmas, no card. Wincing, Tony tells him "I think the Post Office screwed up boss, someone else got your bottle of Jack & you got their honey dust." They discuss that the Post Office irradiates all of the mail and that anything with DNA dies. All the deadly diseases McGee named would be eliminated. Unless, as Gibbs points out, the Post Office screwed up again.

A HazMat team works on cleaning the office and gives Abby the envelope and a letter. The team is in autopsy while Ducky draws their blood for a test to tell if they're infected. Gibbs is chastising McGee saying that he should have given the envelope to him. Kate defends McGee saying that Tony grabbed the letter from McGee, to which Tony replies, "So, it's my bad?" Gibbs wants to start investigating, but Ducky tells him he cannot leave autopsy because it is negative pressure and, therefore, if they are contaminated they won't infect the rest of the building. Gibbs is angry because he says that he has been scrubbed, sanitized and maybe sterilized but Ducky is adamant that Gibbs remain in autopsy.

Tony opened the envelope and may have inhaled some of the powder so he is automatically sent to Bethesda. As they come to pick up Tony, Kate sneezes and is also forced to go with Tony. Because of her cold she is more susceptible to airborne pathogens. At Bethesda, Tony and Kate are quarantined in a special medical area with the relevant equipment.

Tony and Dr. Pitt finds out that they were once rivals on the football field. Kate laments about now having to listen to them. Nurse Emma tells Kate that everyones bloodwork came back negative, except for Tony's. Dr. Pitt also breaks it to Tony that he has the plague. Kate walks over to the bed and says to Tony that only he would get a disease from the Dark Ages. Tony asks her why she was so upset and then realizes that Kate was probably infected too. Kate tells him he was not the only one. Tony asked, "not Gibbs too?" to which she said no, it was just the two of them. She then she informs him that she is going to make his life hell. He asks her how she can make his life any worse than it is now. Kate then returns to her bed to start her treatment. Dr. Pitt follows her. Dr. Brad asked her why she told Tony that she was infected, and she answers, "I don't know". Dr. Brad informs her that he had already told Dr. Mallard that she was not infected. She replies that Ducky would understand why she was doing it, it's Gibbs that won't.

Meanwhile, the rest of the team are working to find out who is behind this incident. Abby & Ducky are examining the letter in Abby's lab. Gibbs wants to get down there and investigate. Ducky tells him that he cannot leave autopsy's negative pressure. Abby is discussing the letter with Gibbs, telling him it’s SWAK—Sealed With A Kiss. She asks Gibbs if he ever got a love letter, and Gibbs returns with the question, ”Does a ’Dear John’ letter count?” Abby replies that she feels sorry for Gibbs. She discovers that the letter has a return address.

Gibbs tells McGee to use Ducky’s PDA to make a note of the address and sends Palmer to get replacement weapons and cell phones. Palmer asks, ”Pistols?” Annoyed, Gibbs yells, ”No, Palmer! Crossbows, if you think they’ll work better!” Gibbs orders Abby to open the letter, and she discovers a very unique, traceable paper with calligraphy writing on it. Meanwhile, McGee exclaims to Gibbs that he can’t find Ducky’s PDA, to which Gibbs yells, ”It’s a pad and pencil, McGee!”

Gibbs asks Ducky to read the letter. As he does, they learn that the powder was a genetically altered strain of Y. pestis, which in Latin means plague. This particular strain would cause pneumonic plague since it was breathed into the lungs. Abby counters that it might not be alive since it needs constant moisture, and McGee remarks that it also would have been irradiated by the post office. Gibbs reminds him that he received honey dust! In reading the letter further, Ducky discovers that the Y. pestis has been altered to be antibiotic-resistant. He also discovers that an antidote will be made available if NCIS will make public the results of a certain case. Suddenly, Abby exclaims, ”Uh oh.” She found a moisture-strip inside the envelope.

McGee discovers that he can’t access the case file they need, so he and Gibbs leave autopsy, despite Ducky’s objections. Ducky's is angry that Gibbs broke quarantine. Abby smiles as she spots Gibbs and Mc Gee arriving in blue containment suits. Using Abby's computer, McGee is able to access the file and finds out that the lead agent was Pacchi. Gibbs tells Ducky to contact Cassie Yates, who was Pacchi's probie at the time. Meanwhile, Abby shows Gibbs the surveillance tape, showing that Tony came back to work to which Gibbs remarks, "Tony always does his best work at night."

Meanwhile, Abby discovers that the SWAK had a lot of lead in it, which protected the bacteria from irradiation. They find out that everybody's bloodwork came back negative, except for Tony's. Cassie arrives and, after looking at the letter, informs Gibbs that she can tell who was probably behind this. It seems the person behind it was a research pharmacist whose daughter had apparently been raped years ago - or so the mother thought; however, it turned out the girl had been into kinky sex and hadn't known how to tell her mother. Her boyfriend had left the hotel to go to the store and been killed in a hit-and-run accident. When the girl was found 2 days later, in her embarrassment at being tied to the bed, she claimed rape. Her mother was angry when sailors were cleared by DNA. The case was never solved.

Cassie tells Gibbs that the person that sent the letter is the CEO of Lowell Pharmaceuticals. They go and confront Hannah Lowell. She tells them that she was the person that sent the letter. Gibbs tells her that she is going to jail for a long time and she is unconcerned. She is dying and doesn't have much longer to live anyhow. She wants them to reopen the case and admit that her daughter was raped. Gibbs tricks her into revealing the microbiologist responsible for the strain and finds him. He tells Gibb that the virus had a 36-hour suicide trigger.

Tony has developed pneumonia and becomes cyanotic. He apologizes to Kate for tormenting her with movie trivia and notices that she is wearing a mask. He asks her why she is wearing a mask and she says it is because she has a cold. He then asks her why she isn't sick and she replies "because I'm stronger" to which he says "are not." He then has a bad coughing bout and Dr. Pitt comes in and sends Kate out. She leaves isolation and bursts into tears in Ducky's arms saying, "He's dying, Ducky." Gibbs walks by them saying "the hell he is" and walks into the isolation area. Dr. Pitt tries to stop him and Gibbs informs him that the virus had a suicide trigger built in and the deadline passed an hour ago. Tony is no longer infectious.

Leaning down next to Tony's head, Gibbs orders Tony not to die. After Tony acknowledges this, Gibbs hands him his new cell phone, advising him to change the number because women keep calling asking for Spanky. As Gibbs leaves, Tony clutches his new phone to his chest.

Afterward, Gibbs meets up with Agent Yates and is told that Hannah started talking and acting crazy on the car ride to the hospital, flashing her breasts and peace signs shouting "Make love, not war!". Hannah's neurologist wanted her in his care and Gibbs said "He can see her here anytime he wishes" at the hospital.

Hannah's daughter is in the hospital waiting room and Gibbs sits down to ask her questions about the night she was raped. She continued to say she didn't recall the events of that night until she slipped and said she told her mom it wasn't a mid shipman. Gibbs inquired further wondering how she could know if she didn't remember what happened that night. Upon further prodding she finally retold the events as they actually happened; she hadn't been raped. It turns out that her lover and her thought it would be funny to tie her up ("It was a joke" she said) and she jokingly told him that she had found the midshipmen downstairs attractive. Then he left her in the hotel room to get something to eat but hadn't returned, after which when too long had passed she started to panic . Unfortunately, she didn't know at the time that the man was killed in a hit-and-run crossing the road. Rather than tell her mother the truth that she was tied up willingly and naked in a room with a man she was presumably physically intimate with, she lied and said she had been raped.

Kate asks Dr. Pitt if she can sleep in the room with Tony and he permits. Nurse Emma tells Kate Tony's asleep. As nurse Emma turns off the lights Kate lies in the bed next to Tony when Tony says "You know this reminds me of the ending of Alien". Kate and Tony share a smile.