After efforts to save the island from invaders in Part 4, some people return in 1975. To protect themselves, they have to integrate into the life here. Meanwhile those who return to the mainland and the island is back and be divided into two groups, one in 2007 (including Jack, Kate, Hugo, Sayid, Sun, Ben, Locke added later) and a rotation in 1975 (Juliet, Sawyer, Jin, …). Who in 1975 try to save the island from the catastrophe, while those in the present continue to discover the secrets of the island…