。。
It must have rained during the night. The road is wet.
Literal
Night [gen-の] interval-during surely rain [subj-が] fell [nominalizer-の][probably-だろう]. Road [subj-が] is-wet [because-から].
きっと ('surely, certainly') combined with ~のだろう ('it is probably the case that') gives a confident inference. Two-sentence deductive structure: first the inferred fact, then the evidence (the wet road) introduced by から. Reverse-cause order is common in conversational reasoning.