Who Stole the Goat? (Who Stole from Whom? Part VII)

One day a goat was stolen. Naturally, Mike, Spike, and Slug were the suspects, and, in fact, one and only one of them was guilty. Each of the three accused one of the others, and Mike was the only one who lied. Was Mike necessarily guilty?

(Source: King Arthur in Search of His Dog and Other Curious Puzzles by Raymond Smullyan)

Consider the two alternatives: first, if Spike stole the goat, then Spike cannot accuse himself, only one of the others, falsely. The same contradiction applies to Slug. Therefore Mike was necessarily guilty.