When a letter sealed with a kiss is delivered to NCIS, Tony assumes it's for him and opens it, only to release a mysterious powder into the NCIS squad room.

Gibbs and the team are forced into quarantine in the NCIS autopsy room pending the results of tests to identify the strange powder. As Abby discovers the substance is a genetically altered version of the plague, Tony and Kate are placed in a bio-hazard isolation ward for fear of exposure. In the clear of the deadly virus, Gibbs and McGee race to track down who sent the letter before the deadly plague destroys one of their team members.