A　3D　nanocavity　array　with　each　unit　composed　of　complementarily　arranged　downward　and　upward　silver　nanoshells　is　reported.　Due　to　the　underneath　planar　gold　film,　the　two　differently　oriented　nanoshells　form　alternatively　closed　and　semi-closed　nanocavities,　respectively,　which　support　dark　and　bright　resonance　modes　of　localized　surface　plasmons　(LSPs).　For　the　symmetric　design　of　such　nanocavities,　mutually　independent　multipolar　LSPs　are　directly　excited　at　normal　incidence,　so　that　the　near-field　coupling　between　them　is　inhibited.　However,　when　the　geometrical　symmetry　is　broken　by　introducing　a　breach　on　a　common　sidewall　of　the　two　adjacent　nanocavities,　strong　interaction　and　consequently　energy　exchange　between　them　become　allowed.　As　a　result,　Fano　coupling　between　the　dark　and　bright　LSPs　is　clearly　observed　as　a　sharp　spectroscopic　resonance.　The　most　important　feature　of　such　a　coupled　dark-　and　bright-plasmon　system　is　that　the　resonance　modes　of　two　different　LSPs　can　be　tuned　separately,　enhancing　largely　the　flexibility　of　the　photophysical　performance　and　controllability.　In　particular,　these　features　enable　promising　applications　of　such　structures　in　single-molecular　detection　through　fluorescence　enhancement,　surface　enhanced　Raman　scattering　(SERS),　and　surface　plasmon　lasers.　SERS　measurements　verify　excellent　performance　of　such　nanocavity　arrays　in　high-sensitivity　detection　of　low-concentration　molecules.