"Lost" returns for its 4th series of action-packed mystery and adventure -- that will continue to bring out the very best and the very worst in the people who are lost.

After Oceanic Air flight 815 tore apart in mid-air and crashed on a Pacific island, its survivors were forced to find inner strength they never knew they had in order to survive.

But they discovered that the island holds many secrets, including a mysterious smoke monster, polar bears, a strange French woman and another group of island residents known as "The Others."

The survivors have also found signs of those who came to the island before them, including a 19th century sailing ship called The Black Rock, the remains of an ancient statue, as well as bunkers belonging to the Dharma Initiative -- a group of scientific researchers who inhabited the island in the recent past.

The band of friends, family, enemies and strangers must continue to work together against the cruel weather and harsh terrain if they want to stay alive. But as they have discovered during their stay on the island, danger and mystery loom behind every corner, and those they thought could be trusted may turn against them.

Even heroes have secrets.