Abstract: A modular, retrofittable, and portable drain water recycle system, for installation into, and removal from, an existing host plumbing system having a host water supply, a sink, a tub or shower, a toilet, and a sewer. The modules include a drain fixture module, a system control module, and a storage tank module, all interconnectable. Gray water is collected from a tub, shower, or sink, by a drain fixture module, and is stored in a storage tank module for reuse in flushing a toilet. The drain fixture module and storage tank module are controlled by the system control module. Capacity can be altered by adding or removing storage tank modules. Different host systems can be accommodated by adding or removing drain fixture modules and system control modules. Drain fixture modules replace or are installed nearby existing P-traps, normally accessible. Storage tank modules and system control modules are of such shape and size as to fit under a typical sink inside the sink cabinet.