Madison Holt is the twin sister of Reagan Holt.
Madison and Reagan were born into the Tomas, with their parents Eisenhower and Mary-Todd. They also had an older brother, Hamilton, and Arnold, a pit bull. They frequently went to Grace Cahill's winter holiday at the Cahill manor. Madison thought she was an old witch though. Madison bodychecked an older boy resting soccer player, and spent more time in the office than in class at school. She head-butted a girl who'll need dentures, and signed up for a new hockey PE elective. She went on a trip to Monument Valley where they jogged around the hotel, and saw the world's largest ball of twine.
After the 'old witch' died, the Holts went to her funeral. Reagan and Madison held Dan Cahill upside down, while Hamilton told him they'd inherit the estate. The twins dropped Dan, and insulted Amy Cahill, who was tounge-tied. Madison but a shovel of dirt on Grace's grave, and the Holts were invited to the will reading. William McIntyre showed a video of Grace saying the Cahill family was famous, amazing the twins who named figures that William confirmed were Cahills. Grace said they could take a million or hunt for the 39 Clues. The Holts were second to decide to join as a team, and got a clue card. The family got in formation, and left, but turned back to trail Amy and Dan. Madison and Reagan jogged in place while Hamilton tried to smoke them out, but the whole mansion ended up burning to the ground.
They trailed the Cahills into Philadelphia at the museum where they set a concussive charge that was activated by the the Starling, triplets on accident. They went to Pairs where they got the Poor Richard's Almanack from Alistair Oh, but Irina Spasky stole it. The Holts rental car was driven in the Seine, and they got an ice cream van to watch the Lucian stronghold. Madison fought Hamilton over ice cream, when Eisenhower told them to form up saying that Amy and Dan entered and got the Almanack (which was an assumption) and they're going to the Île Saint-Louis. On the isle, the Holts stood over Amy and Dan, falling in a hole, asking them if this was the Maze of Bones. Then, a cement truck pulled up with thugs to bury Amy and Dan. Dan lied Amy knew the clue, so Eisenhower saved them. The Holts attacked the thugs, Reagan and Madison slamming a thug into a flower shop across the street. A concussive charge knocked the family out. When they woke they drove to the St. Pierre de Montmarte church, Madison using frozen ice cream as ammunition, going after Amy and Dan, but ran when the police arrived.
The twins got hamburgers in Dijon, France where Madison complained about the excess of mustard, Reagan explaining Dijon was the mustard capital of the world, causing Madison to punch her in annoyance. They got on Amy and Dan's train when it pulled up, and followed them into the mail room where Eisenhower was beaned with a hockey stick. Madison and Reagan held Amy and Dan up, and Eisenhower forced Amy to show them where the KV 617 music sheet was...Saladin having destroyed it. When questioned about being on the train, the Holts jumped off.