A child, or someone remembering being one, sits in a closet drawing ships bound for Jamaica. The closet is a small, private hiding place, and the ships are a way of imagining escape to somewhere far off and sunlit. The drawing feels quiet and a little lonely, but also soothing, as if the speaker is making a route out of a cramped space.
The second stanza shifts the mood. "Late at night" suggests the hour when fears grow, and the speaker says they "had a copy / of my mind." That odd phrase is the heart of the poem. It could mean the drawings are a kind of record of the speaker's thoughts, or that the speaker feels doubled, watching their own mind from outside. Given the title, the "monster" may be that strange, unsettling self. The poem never explains, which leaves a mood of secrecy, imagination, and faint unease.