The Dual-channel Extreme Ultraviolet Continuum Experiment (DEUCE) is a rocket-borne, ultraviolet spectrograph specifically designed to obtain the first flux-calibrated spectra of nearby B stars ϵ and β Canis Majoris (CMa) across the Lyman limit at 912 Å. These observations will help quantify the contribution of such stars to modern ionization in the intergalactic medium. The payload operates in two channels over the 700 to 1150 Å extreme ultraviolet/far ultraviolet (EUV/FUV) bandpass, with resolving powers of 180 (1850) and net effective areas (including slit transmission) of 4  cm² (  .  15  cm²) in its large and small-slit channels, respectively. DEUCE features a large format 200  ×  200  mm² low-noise atomic layer deposition-activated microchannel plate detector of the type baselined for the Large Ultraviolet Optical Infrared (LUVOIR) Surveyor and has demonstrated its flight performance. DEUCE has obtained the first flux-calibrated spectrum of ϵ CMa from 730 to 895 Å. This paper outlines the science objectives, instrument design, calibration, launches, and observations of the mission.