A vast majority of Part I occurs on the Floor of Test. Go here for the arcs or here for the chapters.
The story on the Second Floor starts with the "Deathmatch" Test. The test is simple. Four hundred people start the test. When that number is reduced down to two hundred the test ends. During this test Baam meets his companions Rak and Koon who he teams up with. Also during this test Anak, Ship Leesoo and Hatsu meet and form a team.
All the Regulars from the first test are teleported somewhere else were they take Lero-Ro's test. His test is also simple. Walk through a Shinsoo Barrier that he created. The test is to weed out people with poor Shinsoo Resistance. This is necessary because dense shinsoo is common in the higher Floors of the Tower.
Next came Yu Han Sung's examination in which the Regulars had to rely on a few scant clues to select the "correct" door out of twelve identical doors. The real answer to the test was in fact to open any door before five minutes were up.
After Yu Han Sung's examination, it was revealed that the next group of people killed each other down to three survivors. When Yu Han Sung asked what happened he was told that the district included both an Irregular and a Zahard's Princess.
Yu Han Sung expressed concern that this development would upset Evankhell when he got back. To help deal this problem he decided to host a bonus game called the Crown Game. (Note: Androssi's team also had to pass the Barrier and Door tests, so they could be chosen for the Crown Game.) During the Crown Game a number of things happened including a wager between Anak and Baam, which caused Baam to lose the Black March.
After the Crown Game ended, people had a brief three-day rest before the next test started. During that time Lero-Ro talked to Yu Han Sung about the results of the Crown Game. Lero-Ro says Rachel's team not advancing from the Crown Game was pointless, but Yu Han Sung says that by adding Rachel's team to the game it stopped the most dangerous person to the Tower from advancing so it was worth it.
Also during the rest period, Anak finds out that she can’t get the Black March to work for her. Androssi Zahard reflects on her experience with Rachel and Rachel tells Koon not to tell Baam that she is there.
After the rest, everyone gets together and Lero-Ro explains the rules for the Position Test. The Position Test involves earning points by performing well at your position. The people who earn the most points get to go on to the Final Examination.
During the Position Test, all the Regulars get to know each other more. Some notable things that happen include a fight between Anak and Androssi which almost disqualifies them.
A month passes since the Position Test started and the Regulars must now take part in the final part of the Position Test, the Hide-and-Seek game. Team A, led by Koon, manages to outsmart Quant by making him fall down by using Koon's Lighthouse trick, but is ultimately defeated when Quant unexpectedly catches up with Anak and taps her badge. Team B's test is followed by a series of betrayals, with Androssi thinning the herd, the Spear Bearers defecting which left Hatsu at Quant's mercy and Ho kidnapping and trying to kill Rachel in an attempt to keep Baam from climbing the Tower. This chain of events results in Ho committing suicide and Rachel being crippled, though the team manages to win the game by Androssi taking Quant's badge thanks to the support of Baam's new skill he learned from Quant.
In the following arc, while the Regulars are waiting for their evaluation, Baam returns to his room bearing news about Rachel's condition and determining he'll become her legs instead while climbing the Tower together with all his friends.
While talking to Lero-Ro, Koon's scheme is revealed - finding out about the note sent to Ho, giving Quant a lift with Laure's help causing his own team's loss and tricking him into protecting Rachel. He also asks him if there's a way for the injured Rachel to continue the climb.
Meanwhile, Yugo's true identity is revealed by Yu Han Sung; he is really Lo Po Bia Ren, a member of RED that was tasked to kill Anak and recover the two 13 Month Series weapons. After a short display of their power, Yu Han Sung apologises and tells Ren that he has no intention to interfere with his mission and decides to help him instead.
Shortly before entering the hall, Lero-Ro and the director converse about the true purpose of the Second Floor - filtering the individuals who may bring harm to the Tower. Inside the hall, the names of the passing Regulars are announced. As expected Rachel would be failed due to her injury, therefore Koon raises an objection to her disqualification, asking for permission to receive the final test from the Floor's Second Floor Guardian. The seemingly surprised Yu Han Sung denies him saying that one would have to be an Irregular to able to receive the Guardian's test. With having no other choice left, Baam reveals himself to be an Irregular and manages to get permission to take the Guardian's exam, while Koon tricks everyone into helping Baam and Rachel.
After Ho's short burial, the Submerged Fish Hunt begins. While Koon's plans proceed more or less the way he anticipated, Anak and Androssi who're supposed to be distracting the Bull are lured into Ren's trap. Thankfully they're saved by the timely appearance of Yuri and her team and Ren's puppet is destroyed by Kurudan's attack. In his final moments, Ren informs them that he's sent the Bull to kill Rachel and Baam. Yuri, believing that Baam can handle it, leaves a branch of the Wing Tree to Leesoo along with a message that Urek Mazino would be waiting for Baam on the 77th Floor.
When Baam's bubble is already close to the Net Dolphin Madame's mouth, the Bull appears and attacks him. Seeing as he can't defend and attack at the same time, Baam concentrates all his power into one final attack, killing the Bull with a powerful Shinsoo beam. All seems to be well, when Rachel, standing, pushes Baam out of the bubble and watches as he falls to his death.
While all his friends mourn Baam's death, the whole conspiracy unfolds as Rachel gets a late-night visit from Hwa Ryun. When she leaves the tired 'heroine' to rest, Hwa Ryun sets out to find the 'Night'. As the remaining Regulars are teleported to the next Floor, the guide extends her hand to the injured Baam...
It's revealed that somehow FUG had managed to hide some form of entity in the water cave where the "Submerged Fish hunt" test took place. Its presence may be the true reason for Yu Han Sung to work on the Second Floor.