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Tombstone: What's He Building in There... is a side mission in Marvel's Spider-Man. It is unlocked after completing Tombstone: On the Move. The mission takes place in Harlem.

After defeating the thugs in the previous side mission, Spider-Man is able to track Tombstone's location to the east side of Harlem. At the location are several armed guards. Some can be taken down with sneak attacks if desired. The largest group of Tombstone's gang are near the front, unloading the truck. It might be easier to pick off others in the gang prior to engaging the larger group. Look for a few near lampposts, north of the main entrance. A few others are near shipping containers.

After the initial fighting is over, take a look at the barrels the thugs were unloading. Spider-Man will comment about the chemicals. Right afterwards, another smaller group of thugs will attack. While under the influence of the mysterious chemical, they will be harder to defeat but the chemical does wear off over time. Players having trouble with the fight may want to wait for each one's skin to stop glowing before attacking them, There are also plenty of barrels, bikes, and other items to grab and toss at them which additionally helps to build up focus.

After all the gang members are defeated, return to the first drug-induced biker and grab his drug inhaler. This will start a science mini-game. This one is a little complex in that it takes multiple fragments used together to complete it. The solved puzzle is in the gallery below. The chemical is identified as Diox-3.

To combat the chemical, Spider-Man will need to craft a counter-drug. Look for lab supplies at the back of the truck. Creating the counter-drug will complete the mission.