After one of the 100 return to camp with a virus, getting many of them sick, Octavia, Finn, Jasper, and Raven devise a plot to try to hold off the grounders from war.moreless
An antibiotic is sought to treat Jasper's wounds. Meanwhile, Bellamy and his crew hunt for food.
The surgical interns are presented with an impromptu medical quiz with a chance to assist in surgery as the prize, and Emily and Cassandra emerge as the winners. Emily reluctantly agrees to speak to a group of girls about safe sex after an STD outbreak at a local high school. Meanwhile, Emily struggles to forget about her feelings for Will.moreless
A comet is due to pass by Earth, but its trajectory changes to a collision course, causing widespread panic. When the comet appears to slow down — as if it's coming in for a landing — 4,400 people are deposited in an explosion over Highland Beach.
The returnees — "the 4400" — have been missing from months to years. None have any memory of where they've been, none have aged, and a small number are beginning to manifest paranormal abilities. Tom Baldwin and Diana Skouris, two Homeland Security agents, are appointed to find out what happened to the 4400.