Three children force a classmate to a test of courage to get into their group. During the test, which consists in finding a skull in an abandoned church, the child is swallowed by a chasm that opens on the church floor. The other kids, who had stayed out, rush in the church, look in the deep hole, call their friend, but don't receive answers. The little sister of one of them, who followed them, bursts into the church and lets herself down by a rope, but falls into the abyss, dragging the other kids. All five children find themselves into old caves in search of a way out, helped by Grotto, a creature alike a stalagmite.
