"Do Not Go Gentle into That Good Night" has a defiant mood. The speaker is imploring his aged father to "rage" against impending death rather than to passively accept it. By requesting that his father fight death, the speaker is not suggesting he give into the delusion that he can live forever or that he should be bitter or resentful, but that he should affirm life to the last. As a result, the poem takes on a defiant yet somber mood, with the speaker accepting that death is inevitable while still desperately hoping his father will muster what little energy he can while he is still alive.
The mood is conjured both through the poem's message and its imagery. The speaker tries to get his point across to his father by listing the responses of different kinds of men to death, all of which involve fighting. By observing that wise men, good men, wild men, and grave men all react to encroaching death with defiance, he is hoping to set a standard for his father. The imagery evokes awe-inspiring natural phenomena: "Forked" lightning, blazing meteors, and waves are all used to describe the attitude and deeds of dying men. Unlike more traditional poetry about death, which tends to link such an event with peace and stillness, Dylan Thomas is using bright light and dramatic movement to depict dying as a chance for individuals to shine brightly one last time before that light finally expires.