We microscopically evaluate the excitation spectrum of the 3 He impurity in liquid 4 He at T50 and compare it with the experimental curve at equilibrium density. The adopted correlated basis perturbative scheme includes up to two independent phonons, intermediate correlated states, and the correlation operator is built up with two- and three-body correlation functions. The experimental spectrum is well described by the theory along all the available momentum range. A marked deviation from the simple Landau-Pomeranchuck quadratic behavior is found and the momentum-dependent effective mass of the impurity increases by ;50% at q ;1.7 A 21 with respect to its q50 value. No signature of rotonlike structures is found.