  We present an unitary dispersive model for the $\eta \to 3 \pi$ decay process
based upon the Khuri-Treiman equations which are solved by means of the
Pasquier inversion method. The description of the hadronic final-state
interactions for the $\eta \to 3\pi$ decay is essential to reproduce the
available data and to understand the existing discrepancies between Dalitz plot
parameters from experiment and chiral perturbation theory. Our approach
incorporates substraction constants that are fixed by fitting the recent
high-statistics WASA-at-COSY data for $\eta \to \pi^+ \pi^- \pi^0$. Based on
the parameters obtained we predict the slope parameter for the neutral channel
to be $\alpha=-0.022\pm 0.004$. Through matching to next-to-leading order
chiral perturbation theory we estimate the quark mass double ratio to be
$Q=21.4 \pm 0.4$.
