The basic premise was that any amount of wonderful and amazing adventures that McCay's imagination could come up with could be realised through the dreams of a small boy: Nemo, the hero of all the strips. The last panel in each strip was always one of Nemo in or near his bed waking up, often being scolded by one of his parents or grandparents for crying out in his sleep and waking them. In the earliest strips, the dream event that woke him up would always be some mishap or disaster that seemed about to lead to his death, such as being crushed by giant mushrooms, being turned into a monkey, falling from a bridge being held up by "slaves," or gaining 90 years in age. The adventures leading to these disasters all had a common purpose: to get to Slumberland, where he had been summoned by King Morpheus, to be the "playmate" of his daughter, the Princess.
Sometime during early 1906, Nemo did indeed reach the gates of Slumberland, but had to go through about four months of troubles to reach the Princess. His problem was that he kept being woken up by Flip. Flip is an interesting and subversive character on several levels. Whilst the motto of Slumberland was "Don't Wake Up", one sight of Flip (with his "Wake Up" hat) was enough to take Nemo back to the land of the living, during these early days. Although at first an enemy, Flip went on to become one of the recurring heroes. The others included: Dr Pill, The Imp, the Candy Kid and Santa Claus as well as the Princess and King Morpheus.
The strip was not popular in its time. Most readers preferred the slapstick antics of such strips as Katzenjammer Kids, Happy Hooligan and Buster Brown to the surreal fantasy of Nemo. During the late twentieth century and early twenty-first century, the strip received more recognition. Among the most noticeable of its qualities are: its intricate style and high levels of detail, variation of colours, fast pace of movement from panel to panel and the huge variety of strange characters and scenery.
The strips, along with most of the rest of McCay's works, fall into the public domain worldwide on January 1, 2005. The complete set of Little Nemo strips are available in a single volume: ISBN 3822863009.
A modern-day animated feature film entitled Little Nemo: Adventures in Slumberland was released to movie theaters in 1992, though it was not a box-office hit.