The game starts out with the player choosing one of five planets to travel to. Each planet features a minigame starring one of the Journey band members, with the objective of collecting his musical instrument (or, in the case of Steve Perry, a microphone).
Once the instruments are collected Journey performs a concert. During the concert sequence, an edited, looped version of "Separate Ways" is played through a cassette player inside the machine. The nature of these units mean that very few of the arcade cabinets are still in good working order, which could explain why the game is quite scarce at this point, rarely being seen at arcades. While the concert sequence is being performed, the player takes control of Journey's roadie, a large man whose object is to keep the fans from rushing the stage by placing himself in front of one of three entrances to the stage. Once the fans do rush the stage (which is inevitable, as they move faster and more aggressively as gameplay continues), they run off with the band's instruments, and the game thus starts at the beginning again.
Source: Wikipedia, "Journey (arcade game)," available under the CC-BY-SA License.