1.Nf3 f6 2.e3 e6 3.Nd4 f5

The main line starts with 4.Qh5+ g6 5.Qg5 Qxg5 (or 5.Qh4 Qxh4) 6.Nb5.

Other moves for white: 4.Nb5 Qh4 5.g3 Qc4 6.Bxc4 and 4.Nc3.