During the COVID-19 pandemic in May 2020, detective Benoit Blanc is approached by Helen Brand, played by Janelle Monáe, to look into the suspicious death of her twin sister, Cassandra "Andi" Brand. Just two days prior, Andi had tragically taken her own life, although the news of her demise had yet to become public. Hours before her death, she had emailed Miles Bron ([Edward Norton]) and his group, issuing a threat to expose incriminating evidence that could dismantle Miles' powerful empire.
Helen is convinced of foul play, particularly since Andi had mentioned a crucial piece of evidence hidden in a red envelope that could prove detrimental to Miles. Following a mysterious invitation to a murder mystery weekend hosted by Miles on his luxurious private island in Greece, Helen is urged by Blanc to pose as her sister, believing that Andi's killer will recognize the deception and may act again to conceal their tracks.
Andi was once the CEO of Alpha, a technology firm she co-founded with Miles. After a fierce disagreement over the launch of Klear, a new hydrogen-based fuel that could potentially endanger lives, Miles strategically ousted Andi, leading to a crushing lawsuit—one she ultimately lost. Andi had originally written the concepts that birthed Alpha on a napkin, which Miles later exploited for personal gain, leaving her destitute.
As suspicions mount, Blanc and Helen uncover a tangled web of lies surrounding Andi's former friends: Lionel Toussaint (Leslie Odom Jr.), Claire Debella (Kathryn Hahn), Birdie Jay (Kate Hudson), and Duke Cody ([Dave Bautista]). Each played pivotal roles in their shared history, but now find themselves entangled in potentially devastating consequences tied to Miles.
As the weekend unfolds amidst chaos, tensions escalate. With motives revealed, including threats against Lionel's career and Duke's financial aspirations, Helen digs deeper, suspecting that each member of the group might have played a part in Andi's demise after she discovered the vital napkin.
Blanc, determined to crack the case, disrupts Miles’s murder mystery by solving it before it even begins, giving Helen the opportunity to probe further. Tensions peak during a lavish party when a shocking turn of events leads to Duke's sudden death, igniting panic and fear among the guests.
Determined to unveil the truth, Helen puts her life on the line after an unknown assailant shoots her, only for Andi's journal to save her. The story spirals into a confrontation at Miles's art room, where revelations about his duplicity surface. In a bid to silence her, Miles resorts to desperate measures to destroy the evidence against him, leading to a climactic finale.
Faced with monumental stakes, Helen channels her grief and rage into an explosive resolution that could ruin Miles forever. As the Glass Onion mansion crumbles and the famous Mona Lisa is destroyed, the guests band together to ensure Miles's downfall. Ultimately, as authorities arrive, Helen and Blanc find solace, reflecting on justice served amidst the wreckage.
