Team Underground is a team in Sonic Heroes 2. It consists of Manic and Sonia from Sonic Underground and Tails and Cosmo, who are obnoxiously infatuated - much to the chagrin of Sonic, who is very angry with Tails and therefore they need to keep themselves underground.
Manic and Sonia, being both from Sonic Underground, play the drums and keyboard respectively and therefore some Sound Test noises are played by these two. Sonic rarely provides vocals, as these are usually provided by Mina Mongoose of Team Archaic, but does provide them occasionally.
Team Underground is having a lazy sort of day. Tails and Cosmo are simply laying down, and instead of practicing, Manic and Sonia are busy talking about it, writing music near the Underground/Mystic Ruins North north border (procrastinating, in other words). Suddenly, they stop. A loud noise, coming from outside. They take a look, only to find that some people are lurking and have just entered Gigalopolis. They decide that the two of them cannot be bothered to go down and have a look themselves, so they decide to visit Sonic to get them to deal with it. Action stage Mystic Ruins North begins.
Meanwhile, Tails and Cosmo are doing their own thing, when suddenly Team Rose arrives, asking them if they could have a look at some damage at Gimmick Mountain Zone. They happily oblige and action stage Marble Garden Zone plays.
In the meantime, Manic and Sonia have returned. When they realise that Tails and Cosmo have gone, they hit the roof. This shouldn't have occurred. Their verbal assault continues for some time until Tails and Cosmo return, after which they give one hell of a bollocking to Tails and Cosmo and tell them that they are going to Metropolis to practice with Team Chaotix and that if they want to come, they need to say so now. They oblige; just before they arrive, Team Big arrives and Team Underground also sends them down there as well. While they're doing that, action stage Mystic Ruins Underground plays. At the end, they run into Fiona Fox; much to the chagrin of Cosmo, they stay and chat for a bit as they have not seen each other in years. After that, they play Mystic Ruins South.
When they reach Metropolis, Manic and Sonia get straight into practice. Whilst they're up to that, Tails and Cosmo notice that there is some damage to the exterior, which they set about fixing. After a while - at the end of a song - Tails and Cosmo come bursting into the booth, yelling that everything's gone dark. When they all look out there, it'sn't dark any more, but they can visibly see the Egg Carrier. Team Chaotix decide to stay in case they get any more instructions from their anonymous source, but Team Underground decide not to; they decide to run the Tornado up. They go back through Mystic Ruins South as an adventure field and pick up a power-up each; Tails picks up a Rhythm Badge, Cosmo picks up a Magic Magnolia and Manic and Sonia both pick up a pair of Light Speed Shoes.
Mystic Ruins Underground is played in the form of a cutscene, in which we quite audibly hear Fiona Fox giving Team Chaotix their next mission. The scene then cuts to Mystic Ruins North, where Tails gets out the Tornado in a scene similar to the opening of Sky Chase Act I. Action stage Sky Chase Zone is played, and once on the ship, action stage Sky Deck Zone is played.
The next place we see them is on the deck of the Egg Carrier itself. The next thing we see, however, is Chaos Emerl 6. As this is the first time any of them have seen Chaos Emerl in this carnation, groans are generated by the entire team. It is then fought.
Straight afterwards, the team try to get back on to the Tornado to try to get back to the Underground. That isn't going to happen; when they get there, Chaos Emerl had 'noshed' the Chaos Emerald inside the Tornado and it wasn't going to go anywhere. Literally, we see the Egg Carrier exploding and TCMS flying off the Egg Carrier. In a stroke of luck, we see them landing in Mystic Ruins North; we then see them go back into Underground Zone, we then see them shooting the shit with Team Rose, them going, Team Underground talking amongst themselves for a bit and then going to sleep and the cutscene ending.