In a pair of papers, we construct invariants for smooth four-manifolds equipped with 'broken fibrations'-the singular Lefschetz fibrations of Auroux, Donaldson and Katzarkov-generalising the Donaldson-Smith invariants for Lefschetz fibrations.The 'Lagrangian matching invariants' are designed to be comparable with the SeibergWitten invariants of the underlying four-manifold; formal properties and first computations support the conjecture that equality holds. They fit into a field theory which assigns Floer homology groups to three-manifolds fibred over S 1 .The invariants are derived from moduli spaces of pseudo-holomorphic sections of relative Hilbert schemes of points on the fibres, subject to Lagrangian boundary conditions. Part I-the present paper-is devoted to the symplectic geometry of these Lagrangians.53D40, 57R57; 57R15