The Scotch Opening or Scotch Game is signified by 3.d4 after the regular 1.e4 e5 2.Nf3 Nc6. The best theoretical line after that for black is to play 3....exd4. 4.Nxd4 is the main line after 3...exd4. Black has 2 options after 4.Nxd4 is played. They can play the Classical Variation by playing 4.... Bc5 or they can play the Schmidt variation of 4....Nf6. Both ways work for black and any other move besides those two could be deemed as inaccurate.
If 4...Bc5 is played, White has 5.Nxc6, 5.Be3, or 5.Nb3. After 5.Nxc6, play almost always continues 5...Qf6 (Black does not lose a piece on c6 because he is threatening mate with 6...Qxf2) 6.Qd2 dxc6 7.Nc3. On 5.Be3 play almost always continues 5...Qf6 6.c3 Nge7 7.Bc4 7...0-0 (7...Ne5 is more often played here.) 8.0-0 Bb6 where the position is roughly equal. On 5.Nb3 play almost always continues 5...Bb6 6.a4 a6 7.Nc3. Another plan for White is to play 6.Nc3, followed by Qe2, Be3, h4 and 0-0-0.
If 4.... Nf6 is played then White has 5.Nxc6 or 5.Nc3. After 5.Nc3 almost always played is 5...Bb4 6.Nxc6 bxc6 7.Bd3 d5 8.exd5 cxd5 9.0-0 0-0 10.Bg5 c6. After 5.Nxc6 bxc6 6.e5 Qe7 7.Qe2 Nd5 8.c4 is also very common.