Becky has hosted a picnic for her friends. During the picnic, which is at the "woody hollow", the group went to explore McDougal's Cave. Like all children, they are fascinated by the mystery of the cave and excited by the sense of discovery in exploring the cave. Here is a passage from Chapter 31 describing their excursion:
They tripped along the murky aisles with the rest of the company, visiting the familiar wonders of the cave—wonders dubbed with rather over-descriptive names, such as “The Drawing-Room,” “The Cathedral,” “Aladdin's Palace,” and so on. Presently the hide-and-seek frolicking began, and Tom and Becky engaged in it with zeal until the exertion began to grow a trifle wearisome; then they wandered down a sinuous avenue holding their candles aloft and reading the tangled web-work of names, dates, post-office addresses, and mottoes with which the rocky walls had been frescoed (in candle-smoke). Still drifting along and talking, they scarcely noticed that they were now in a part of the cave whose walls were not frescoed.
While the rest of group stays near the opening and easily find their way out, Tom and Becky wander into the depth of the cave. They explore windy and twisting passages, and eventually end up getting scared by a bunch of bats. After running from the bats in panic, the two are lost and have no idea how to find their way out again.