Finn and Clarke's new relationship becomes a little more complicated. Octavia is knocked out and abducted by a Grounder.
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.