Yukio Tanaka, known as "Koyuki" by his friends, is a regular 14-year-old Japanese boy starting eighth grade in junior high school. His boring life is changed when he saves an odd-looking dog, named Beck, from some kids. Beck's owner turns out to be an emerging rock musician, 16-year-old Ryusuke Minami, who soon influences Koyuki to start playing the guitar. The story focuses on the trials and tribulations of their rock band named Beck, and Koyuki's relationships with its members, in particular Ryusuke and his 14-year-old half-sister Maho.
After hanging out with Ryusuke and seeing him play with his former band, Koyuki slowly becomes interested in Western rock music. Ryusuke gives him a guitar, but when Koyuki breaks it, Ryusuke tells him never to speak to him again. At the same time, Ryusuke forms his new band Beck, with vocalist Tsunemi Chiba, bassist Yoshiyuki Taira, and Togo, the drummer from his previous band. Koyuki begins working for, and learning guitar from, 44-year-old Kenichi Saitou in exchange to have the guitar fixed.
He reunites with Ryusuke a year later, and begins to rehearse with Beck. Koyuki then makes friends with his classmate Yuji "Saku" Sakurai. When Togo leaves the band, Ryusuke has Koyuki and Saku join Beck as support musicians, becoming full members only when the band hears Koyuki sing. Eventually Beck releases their first album, which gets released on an independent record label in the United States, under the band name Mongolian Chop Squad. After gaining popularity from their US album and Koyuki being in an internationally screened concert documentary, Beck earns a spot at the music festival Grateful Sound 5, where they put on the most talked about show of the whole festival. (The live-action film adaptation ends here.) However, circumstances cause them to part ways, making it their last performance.
Finding life tedious without being in Beck, Koyuki slowly gets the members back together, except Ryusuke, whose whereabouts are unknown. They perform a few shows as a quartet, before getting an offer to tour the US based on their Grateful Sound 5 performance. After Koyuki and Saku drop out of school to do the tour, Beck heads to America. But after several bad performances, they are about to get kicked off the tour before reuniting with Ryusuke in Seattle. (The anime adaptation ends here.) The rest of the tour is a hit and they end up appearing on national TV before heading back to Japan.
After releasing two singles, Beck goes on a nationwide tour of Japan and earn a spot at Grateful Sound 7. However, they are later cut from the lineup. They slowly bounce back after forming a tour with several similar-sounding bands, get signed to a popular British indie record label, and start recording their first full album. The now-famous director who created the concert documentary Koyuki once appeared in ends up directing their first music video. Their album and music video do well both in Japan and England, earning them numerous magazine articles in both countries. After another nationwide tour of Japan, they do a short tour of England, including a spot at the relaunch of the legendary Avalon Festival. The band then signs to a major international record label and records their major debut album in New York. With the album hugely successful worldwide, they tour Japan and America extensively, and the series then ends with Beck headlining the main stage at Grateful Sound 9.