Go toward the east end of Greater Boston (downtown) and look for one of the tallest towers. It's white with bits of yellow, and it's hard to miss. This is Trinity Tower, and it's emitting a radio signal. Tune into it with your Pip-Boy to initiate the mission. It turns out that Rex Goodman, the DJ of broadcast station WRVR, is in need of rescue from some Super Mutants.
Given the narrowness of the building, the path upward is very straightforward, especially when two elevators will shorten the journey. You just have to contend with a building's worth of Super Mutants. Rex and his Super Mutant friend, Strong, are locked in a room protected with a master-level lock. Either have that lockpick level at the ready or look for a key in a nearby steamer trunk. After you've opened the door, now is the time to make your way down.
The path downward will partly involve using two descending exterior platforms, the kind that can be used for window washing. You can help Rex and Strong pick off the remaining Super Mutants as the contraption lowers you. The quest is complete when you make it to the bottom and converse with Rex. As a bonus, you'll now have Strong as an available companion.