Iris welcomes an enigmatic ally to their community while Hope questions the visitor's motives. A message upends the sisters' worldview, forcing them to decide between the safety of their home and the uncertainty of the world beyond.
Dr. Connor Rhodes is reunited with his sister as he treats a family friend; Dr. Natalie Manning must persuade a young musician to undergo a life-saving surgery that could destroy her hearing.
A patient refuses medication for her pain; Dr. Halstead treats a former high school classmate; a pregnant woman has a big reason not to deliver her child.
An old friend's heart problems reveal a bigger issue than anticipated; Devon treats a suicidal woman who he thinks might be a good candidate for deep brain stimulation; Billie and Trevor have an emotional reunion.
While the group works together to cross the Mississippi River, a plot to steer the teens back home hits a snag. A member of the group must overcome past trauma to save the others from an approaching threat.