1.Nf3 f6 2.Nd4 Nh6 3.Nf5 Nxf5 4.Nc3 c6 5.e4

The main move is 5...g6.

Other lines:

5...e6 6.Nb5 (or 6.Bb5 cxb5 7.Nb5) cxb5 7.Qh5 g6 8.Qb5 Nc6 9.Qb6,

5...Qb6 6.d4 Qxb2,

5...Qa5,

5...a6,

5...h5,

5...d5 is weak, 6.Nb5 Bg4 (6...cxb5 7.Bb5 Nc6 8.d4).

5...d6 is losing due to 6.Nd5 cxd5 7.Qh5 g6 8.Qb5 Nc6 9.Qb6 axb6 10.Bb5 Kf7 11.Be8 Ke6 12.e5.