Describe the ordeal faced by Dorothy and the Lion in the poppy field.

After getting through the dark forest relatively unscathed, Dorothy and her friends find a beautiful place by the river to spend the night. The next day, it seems like Dorothy and her friends are not going to experience any hardships. Dorothy eats a breakfast of peaches and plums, and the Tin Woodman makes a raft that will take them across the river to the Emerald City. Unfortunately, they run into more problems while they are going across the river. The Scarecrow falls off the raft, and none of his friends can help him. Finally, a stork flies by, and he picks up the Scarecrow and flies him to dry land.
The friends are exhausted from their ordeal, so Dorothy lies down in a field of poppies, despite the fact that the Tin Woodman and the Scarecrow warn her not to. The longer someone sleeps in the poppies, the longer they stay asleep.
The poppies do not affect the Tin Woodman or the Scarecrow, because they are not living beings. The Lion starts to fall asleep, so the Tin Woodman and the Scarecrow tell him to get out of the poppy field quickly, and they will be right behind him, carrying Dorothy.
The Lion does, however, fall asleep right at the edge of the poppy field, and he is too heavy for the Scarecrow and Tin Woodman to pick up, so they have to leave him there.

In chapter 8 of L. Frank Baum's The Wonderful Wizard of Oz, Dorothy and her companions are faced with the challenge of traveling across a field of poppies. When they first enter into the field, each companion reacts to it in a different way:

"Aren't they beautiful?" the girl asked, as she breathed in the spicy scent of the bright flowers.
"I suppose so," answered the Scarecrow. "When I have brains, I shall probably like them better."
"If I only had a heart, I should love them," added the Tin Woodman.
"I always did like flowers," said the Lion. "They seem so helpless and frail. But there are none in the forest so bright as these."

Here the Scarecrow and the Tin Woodman are reminded of their perceived shortcomings, while Dorothy and the Lion observe and enjoy the sight and scent of the flowers. Thankfully, the Scarecrow and Tin Woodman are not capable of breathing, as it is the two of them that manage to help to save Dorothy and the Lion from the harmful effects of the flowers. Baum writes:

Now it is well known that when there are many of these flowers together their odor is so powerful that anyone who breathes it falls asleep, and if the sleeper is not carried away from the scent of the flowers, he sleeps on and on forever. But Dorothy did not know this, nor could she get away from the bright red flowers that were everywhere about; so presently her eyes grew heavy and she felt she must sit down to rest and to sleep.

At this point the Scarecrow tells the Lion to try to run out of the field before the Lion also is affected, and then the Scarecrow and Tin Woodman make a chair out of their hands and begin to carry Dorothy from the field. However, they eventually come to the place where the Lion has passed out, and they are unable to lift him. They realize they cannot help him, at which time "they carried the sleeping girl to a pretty spot beside the river, far enough from the poppy field to prevent her breathing any more of the poison of the flowers, and here they laid her gently on the soft grass and waited for the fresh breeze to waken her."By chapter's end, it seems that the companions have lost one of their own. However, in the very next chapter, Baum employs a moment of deus ex machina in the form of field mice, who help to save the Lion and allow the companions to continue their journey.