In this paper, we study the asymptotics of polynomials orthogonal with respect to the varying quartic weight ω(x) = e −nV (x) ,w hereV (x) = Vt(x) = x 4 4 + t 2 x 2 . We focus on the critical case t = −2, in the sense that for t −2, the support of the associated equilibrium measure is a single interval, while for t< −2, the support consists of two intervals. Globally uniform asymptotic expansions are obtained for z in three unbounded regions. These regions together cover the whole complex z-plane. In particular, in the region containing the origin, the expansion involves the Ψ function affiliated with the Hastings- McLeod solution of the second Painleve equation. Our approach is based on a modified version of the steepest-descent method for Riemann-Hilbert problems introduced by Deift and Zhou (Ann. Math. 137 (1993), 295-370).