The poem "Some One", by Walter de la Mare, is very reminiscent of his poem "The Listeners." (A link to "The Listeners" can be found below.)
In his poem "Some One," Walter de la Mare shows a speaker who hears a knocking on his door. Upon opening the door, the speaker does not find anyone there. Instead, the speaker is met by sounds of nature:
Only the busy beetle
Tap-tapping in the wall,
Only from the forest
The screech-owl's call,
Only the cricket whistling
While the dewdrops fall.
It is here where the speaker realizes that he (assumptive based upon the gender of the poet) that no one is actually at his door. Confused, the speaker states that he has no clue who has knocked on his door.
The final line of the poem allows for the reader to recognize the sound of knocking by repeating the words "at all" three times. this is similar to how one would knock on the door.
As for analyzing the poem's theme (as part of the summary), the theme of the poem seems to be somewhat mysterious--like the fact that no one is at the door. the sounds of the night add to this mystery. Was it the beetle knocking that the speaker heard, or something more? This is left up to the reader's interpretation.
One could justify that the use of nature in the poem is meant to remind the speaker that he is never really alone. Instead, he is surrounded by things which go bump in the night- the beetles, the owls, and the crickets.