  A weak-doublet with hypercharge $-3/2$ is one of only a handful of fermions
which has a renormalisable interaction with Standard Model fields. This should
make it worthy of attention, but it has thus far received little consideration
in the literature. In this paper, we perform a thorough investigation of the
phenomenology which results from the introduction of this field, $F$. After
expressing the model in terms of its effective field theory at dimension-6, we
compute a range of electroweak and leptonic observables, the most stringent of
which probe up to $M_F \sim 300$ TeV. The simplicity of this scenario makes it
very predictive and allows us to correlate the different processes. We then
study how this new fermion can connect the SM to various simple but distinct
dark sectors. Some of the most minimal cases of $F$-mediated dark matter (DM)
involve frozen-in keV-scale scalar DM, which may produce x-ray lines, and
frozen-out TeV-scale fermionic DM.
