Underground.A new Mini-con appears and the Autobots and Decepticons go after it. The signal comes from a large city. In order to obtain the new Mini-con the two races will have to go underground. The Decepticons send their Minis to investigate the tunnels. The kids together with their Mini-cons do the same. During their quest they get into a derelict subway station. Here they find a Mini-con panel which is walled up in the ceiling (meanwhile somebody watches them). They succeed in pulling the panel out and decide to return to the ’Bots as soon as possible. While going through a tunnel the kids spot somebody (seemingly a 'Con) chasing them. They run away but are faced with the Decepticons' Minis. The kids seek safety in flight again. Now they are in a strange natural tunnel. Rad sends Lazerbeak to Optimus Prime for help. However, the Decepticons find their shelter well before the Autobots have time to get there. Suddenly an unknown Transformer comes to children's aid and fights the Decepticons. Just in this time the tunnel collapses and buries both the kids, the Minis and their defender. When Optimus Prime and his guys come at last it seems to them that it's all over. Cyclonus takes the new Mini-con, and the 'Cons warp out. But it appears very soon that everybody is alive. The mysterious Transformer turns out to be an Autobot warrior named Smokescreen. All the 'Bots are happy to see their old chap again.