A person goes to the movies and notices something missing: the hand that used to hold theirs. The speaker never says outright that a partner is gone, whether through a breakup, a death, or just distance. The poem leaves that open. All we get is the quiet absence, and the word "again" suggests this isn't the first time the speaker has felt it.
What stands out is the speaker's odd, almost detached question. They wonder whether this loss is "part of some kind of plan" or whether it "just spontaneously happened." It's a way of searching for meaning. If there's a plan, the pain has a reason. If it simply happened, it was random, and that may be harder to accept. The tone is flat and understated, but the longing sits underneath. The sadness comes from how plainly the speaker describes something so small: one hand, one seat, one ordinary evening.