  Given a link in the three-sphere, Z. Szab\'o and the second author
constructed a spectral sequence starting at the Khovanov homology of the link
and converging to the Heegaard Floer homology of its branched double-cover. The
aim of this paper and its sequel is to explicitly calculate this spectral
sequence, using bordered Floer homology. There are two primary ingredients in
this computation: an explicit calculation of filtered bimodules associated to
Dehn twists and a pairing theorem for polygons. In this paper we give the first
ingredient, and so obtain a combinatorial spectral sequence from Khovanov
homology to Heegaard Floer homology; in the sequel we show that this spectral
sequence agrees with the previously known one.
