  Given a general K3 surface $S$ of degree $18$, lattice theoretic
considerations allow to predict the existence of an anti-symplectic birational
involution of the Hilbert cube $S^{[3]}$. We describe this involution in terms
of the Mukai model of $S$, with the help of the famous transitive action of the
exceptional group $G_2(\mathbb{R})$ on the six-dimensional sphere. We make a
connection with Homological Projective Duality by showing that the
indeterminacy locus of the involution is birational to a $\mathbb{P}^2$-bundle
over the dual K3 surface of degree two.
