Good Bye, Mr. Jack is a high-energy electronic track by Japanese composer sasakure.UK, originally released on his 2015 album ロストピリカ (Lost Pirica). The song joined maimai FiNALE on May 10, 2019, in the GAME & VARIETY category, and has been carried forward through subsequent DX revisions with periodic level adjustments. Clocking in at 180 BPM with a duration of 2:22, the track features sasakure.UK's signature blend of intricate synth melodies, driving percussion, and glitch-inflected production, hallmarks of his work across the rhythm-game and doujin music scenes. The song holds a layer of self-referential wordplay: its title, Good Bye, Mr. Jack, echoes the name of the chart designer Jack, who crafted both the Expert (level 12+, constant 12.9) and Master (level 13+, constant 13.9) charts. This marks the second time Jack has solo-designed both of the top two standard difficulties in maimai, following the precedent set by Justified. The Master chart, with 933 total notes and a 13.9 difficulty constant, sits among the more demanding 13+ offerings of its era. Both standard charts have seen difficulty reclassifications across version updates, reflecting the evolving difficulty calibration of the maimai DX series.