  In $26+1$ space-time dimensions, we introduce a gravity theory whose massless
spectrum can be acted upon by the Monster group when reduced to $25+1$
dimensions. This theory generalizes M-theory in many respects and we name it
Monstrous M-theory, or M$^{2}$-theory. Upon Kaluza-Klein reduction to $25+1$
dimensions, the M$^{2}$-theory spectrum irreducibly splits as
$\mathbf{1}\oplus\mathbf{196,883}$, where $\mathbf{1}$ is identified with the
dilaton, and $\mathbf{196,883}$ is the dimension of the smallest non-trivial
representation of the Monster. This provides a field theory explanation of the
lowest instance of the Monstrous Moonshine, and it clarifies the definition of
the Monster as the automorphism group of the Griess algebra, by showing that
such an algebra is not merely a sum of unrelated spaces, but descends from
massless states for M$^{2}$-theory, which includes Horowitz and Susskind's
bosonic M-theory as a subsector. Further evidence is provided by the
decomposition of the coefficients of the partition function of Witten's
extremal Monster SCFT in terms of representations of $SO_{24}$, the massless
little group in $25+1$; the purely bosonic nature of the involved
$SO_{24}$-representations may be traced back to the unique feature of $24$
dimensions, which allow for a non-trivial generalization of the triality
holding in $8$ dimensions. Last but not least, a certain subsector of
M$^{2}$-theory, when coupled to a Rarita-Schwinger massless field in $26+1$,
exhibits the same number of bosonic and fermionic degrees of freedom; we cannot
help but conjecture the existence of a would-be $\mathcal{N}=1$ supergravity
theory in $26+1$ space-time dimensions.
