  We study the moduli space of pairs consisting of a smooth cubic surface and a
smooth hyperplane section, via a Hodge theoretic period map due to Laza,
Pearlstein, and the second named author. The construction associates to such a
pair a so-called Eckardt cubic threefold, admitting an involution, and the
period map sends the pair to the anti-invariant part of the intermediate
Jacobian of this cubic threefold, with respect to this involution. Our main
result is that the global Torelli theorem holds for this period map; i.e., the
period map is injective. To prove the result, we describe the anti-invariant
part of the intermediate Jacobian as a Prym variety of a branched cover. Our
proof uses results of Naranjo-Ortega, Bardelli-Ciliberto-Verra, and
Nagaraj-Ramanan, on related Prym maps. In fact, we are able to recover the
degree of one of these Prym maps by describing positive dimensional fibers, in
the same spirit as a result of Donagi-Smith on the degree of the Prym map for
connected \'etale double covers of genus 6 curves.
