  The ability to quickly detect transient sources in optical images and trigger
multi-wavelength follow up is key for the discovery of fast transients. These
include events rare and difficult to detect such as kilonovae, supernova shock
breakout, and "orphan" Gamma-ray Burst afterglows. We present the Mary
pipeline, a (mostly) automated tool to discover transients during high-cadenced
observations with the Dark Energy Camera (DECam) at CTIO. The observations are
part of the "Deeper Wider Faster" program, a multi-facility, multi-wavelength
program designed to discover fast transients, including counterparts to Fast
Radio Bursts and gravitational waves. Our tests of the Mary pipeline on DECam
images return a false positive rate of ~2.2% and a missed fraction of ~3.4%
obtained in less than 2 minutes, which proves the pipeline to be suitable for
rapid and high-quality transient searches. The pipeline can be adapted to
search for transients in data obtained with imagers other than DECam.
