The poem begins in a light, almost childlike voice. The speaker is "off" to play the first horse on Mars, then "off" to be a dove. These are odd, imaginative images, the kind of thing you might say as a joke or dream up on a whim. Mars suggests escape to somewhere impossibly far away, and the dove suggests peace, lightness, and flight.
Then the tone drops. "I took / some pills" is flat and plain, and the short broken line makes it feel like a pause or a held breath. Reading it back, the earlier lines look different. "Off" may mean more than leaving for an adventure; the poem seems to hint at someone trying to leave life itself, or at least to get away from pain. The fantasy and the pills together suggest a mind reaching for relief.
The mood is unsettling because of that contrast. The poem stays calm and quiet, and that restraint makes the final lines hit harder.