The aim of this paper is to explain, from a systematic and unified point of view, the following two facts: (i) sentences containing be going to (be going to -sentences) can easily go with adverbials of future time reference such as tomorrow , while sentences containing be about to (be about to -sentences) basically cannot, and (ii) in narrative texts be going to -sentences in the past tense tend to be used to indicate fulfillment of the situation expressed by the infinitive, whereas be about to -sentences in the past tense tend to be used to imply unfulfillment of it. To provide an explanatory basis for these two issues, the present paper formulates the temporal structures of be going to - and be about to -sentences. It is shown that the temporal structures interact with such factors as metonymy and the nature and grammar of narrative texts, resulting in the observed differences between the two types of sentences.