This is best introduced during your existing campaign as several minor news stories in the background about increased reports of incidents in the city metro system. It should culminate with the breif radio clip about local psychics sensing things amiss from the routes that also have the highest concentration of rider complaints issued in the last month, by 30% more than any other routes.
If all else fails, they didnt connect the reports, and end up chasing a supervillain quarry in civilian guise onto a crowded subway train, having to follow in their civilian (or nonsuperhero alias) identities as well.
Either way, the PC's are on the subway train on Route B-6, sometime around 6:00 pm in the evening.
If the PC's are chasing a supervillain in civilian guise, they lose him at the next stop, and the doors slam shut before theyre able to go out and get him. If theyre investigating, superhero guise or otherwise, everyone gets off the train except the driver, to let them do their job.
Within 4 minutes out of the 10 to the next stop, the train breaks down right there in the tunnel. The lights flicker and go out. If this is option A, the civilians are probably panicking and making life hard for the disguised supers. If this is option B, the coustumed PC's most likely suspect an attack.
Within a minute the train begins going again and the lights flicker back to life. A munching can be heard in the car directly in front or behind of the one the PC's are in. Investigating, probably suspecting the worst... they find the car totally empty except for The Subway King, who is happily munching on cheap bagged snackfood.
Dressed in wellworn but trendy clothing, unshaven but charming in an eccentric way, the young man called the Subway King has the unique metahuman ability to command the metro trains of the city.
When comfronted with any questions the happy go lucky King will simply order the train to open its doors as another passes, opening its doors as well and simply hopping from one to the other. The PC's may follow, but will cause a panic among the next train's occupants, thus hindering their following. The Subway King is cunning and inventive, and can go on subway hopping for hours on end.
If the PC's keep following out of attricion, or use their powers to forcefully stop him, the Subway King will simply whistle for a metro train to come along and hop aboard, or if more desperate, try to get one to run over the heroes! He doenst want to fight, but is very desperate for escape.
The best way to catch him is to force him off a train onto one of the stops, and get between him and the tracks. At this point the real battle begins.
What they thought was just an annoyance that needed explaining turns to be lethal when the carefree Subway King summons a giant larval creature that bursts through the cement of the underground metro stop. The King commands it, but with glazed eyes and without speaking at all, as if he were under the thrall of something.
Now the PC's must stop the creature from escaping or hurting bystanders at the stop, which can also be done by knocking out the King, which sends the larval creature into a catatonic state.
Regardless, when the supers have finished the battle, the scientists wth the police supers unit or supporting lab organization will discover and extract an alien creature from within the brain of the Subway King. The now confused King, identified by police as Shaun Patterson, has been under the control of an alien parasite, moving around and acting with his usual mannerisms, but totally not in controll of his thoughts.
The giant larval creature has been using Shaun as a homing point to follow the underground metro tracks, tunnelling beneath them. Within a month, the entire subway would of been ready to collapse, falling into the mouths of awaiting parasite larvaes. With the defeat of the creature by the PC's, national superhero groups move in and remove the rest of the alien manifestation, and the PC's get their names in a front page/top story on the local news.
Escape From Monster Island
Freedom City 90210
"Look out boys, it's Astro City's greatest hero!"
"Hey, I dont see Samaritan anywhere..."