Mark Lee

With time running out before their window home closes, the survivors hatch a final escape attempt, but that plan is put into question when they receive a stark warning from Gavin that it will end in disaster, forcing Eve to make an impossible choice.

When plans to infiltrate the building go awry, Gavin comes face-to-face with its enigmatic leader; Eve, Levi, Sam and Izzy are forced to rely on a former adversary to lead a rescue attempt; Josh and Riley enjoy some fun before a startling turn.

Desperate to reunite with her son, Eve follows a lead that brings her face to face with a ruthless prehistoric group; Gavin, Izzy and Ella question whether they're fit to survive in 10,000 B.C.; Josh and Riley wake up in an unfamiliar time.

With Gavin on the verge of dying, Eve and her family make a desperate attempt to set things right -- even as an unexpected twist threatens to tear them apart forever; Veronica reveals a secret that sends her relationship with Lucas into jeopardy.

With news of one last sinkhole opening, Gavin, Izzy and Dr. Nathan race to Seattle to launch a final rescue effort before it's too late; Eve embarks on a perilous journey to send a young boy though a portal in order to save her family.

When the clearing and fort unite to celebrate the union of Ty and Paara, the invitation of a controversial guest threatens to upend the festivities; Lucas offers Veronica an unexpected strategy to unravel her captor's odd connection to 10,000 B.C.