A novel metal-free double-annulation cascade for the construction of unusual fused heterocyclic systems is described. This simple protocol enables the sequential assembly of two rings in one pot from two simple precursors. Acidic conditions promote the condensation and the intramolecular alkynyl Prins reaction of an enyne or arenyne alcohol with a cyclic hemiaminal to form a five-, six-, or seven-membered oxacycle followed by a seven- or eight-membered azacycle. In this transformation, chemical complexity is rapidly generated with the formation of three new bonds (one C-O, one C-C, and one C-N) in one synthetic operation. The strategy is modular and relatively general, providing access to a series of unique fused bicyclic scaffolds.