A serial killer taunts the CSIs by leaving them cryptic hints, and the longer it takes them to decode these puzzles, the higher the death toll rises.

The episode opens with each of the CSI’s receiving an unstamped, apparently hand-delivered piece of mail containing pieces of a transparency. Once assembled, an image of a woman in the crosshairs of a rifle scope are clear, along with an address. Upon responding to the address, police find that a woman has been fatally wounded by a gunshot to the head from a sniper rifle. In the background, her daughter is singing the well-known children’s rhyme, “Ring around the rosie”.

A series of paychecks found in the house lead the CSI’s to Dade University, where the woman had participated in a research experiment similar to those performed in the classic Milgram experiments (in which participants are ordered to shock and, eventually, electrocute a person who is, unbeknownst to them, not being shocked but actually helping to perform the experiment).

The CSI’s initially clear the researcher and her faculty mentor of any wrongdoing. Shortly afterward, they receive another letter, delivered to the dead woman’s address, which contains an image of the Faculty Pool at Dade University, where a second victim has been drowned. A message found at the second crime scene suggests to them that they have a serial killer, and that the killer is targeting the psychology department—where the researcher already cleared had worked, and recently had been dismissed, as a result of experimental fraud.

The CSIs identify the killer’s next target as likely to be another member of the PhD committee of the dismissed student, and offer to protect the remainder of her committee. One of the protected men is then set afire by a cologne bottle spiked with potassium, which he uses immediately after taking a shower. On the back of the bottle is a reference to an academic article published by the last remaining committee member, which focuses on public reactions to infectious disease.

Certain of the identity of their suspect, they arrest the student who had been dismissed, only to find her particularly uncooperative. She provides a solid alibi for the time of the second murder, however, and the CSIs are forced to search for a different suspect. In the process, they discover that the student’s mentor has recently been denied tenure, and that the same group of people who comprised the student’s committee also refused his tenure. Evidence from his office links him with the delivery of the transparency that set all of the episode’s events into motion.

The CSI’s immediately proceed to arrest him, and Horatio Caine asks him why he sent the letters to the CSI’s in the first place. His answer is not shown, but while being lead away an envelope falls to the floor, labeled Horatio Caine, Confidential. The next scene is cut with scenes of the CSI’s and technicians at the crime lab apparently coughing and choking. The letter is opened to reveal the last line of the children’s rhyme from earlier in the episode, stating, “They All Fall Down.”

At the end of the episode, we see all of the techs and CSI’s at the lab falling to the ground, apparently succumbing to respiratory distress, likely a result of infection with pneumonic plague. The episode ends with Delko re-entering the building and finding his fallen friends, and calling for help while holding Calleigh.

The speculation that the CSI’s have been infected with plague is supported by two facts during the episode. The first is the repetition of the children’s rhyme “Ring around the rosy”, which is speciously thought to refer to bubonic plague, and the revelation of a letter containing the final verse given to Horatio Caine at the end of the episode. The second fact is that the perpetrator has written several articles about distress during outbreak of serious disease, and a particular mention is made, after the death of a colleague, about bubonic plague. The respiratory symptoms are consistent with pneumonic plague exposure, although the simultaneous collapse of nearly the entire staff is more consistent with an airborne nerve agent than any infectious disease, which would show patterns of collapse consistent with duration of infection and personal susceptibility.

“All Fall Down” is the twenty-fourth and final episode in the eighth season of the American crime drama CSI: Miami. The episode aired on May 24, 2010.

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