In 2050, the Earth faces a catastrophic threat as dark energy destabilizes the sun, heralding a new ice age and putting all life at grave risk. The solution is an experimental stellar bomb that has a mass comparable to Manhattan Island. This bomb is meant to eradicate the dark energy and restore the sun's stability. The mission is entrusted to the Icarus I, a prototype spacecraft equipped with state-of-the-art heat shielding and an advanced artificial intelligence (AI). Unfortunately, the mission ends in tragedy.
Seven years later, the crew of the Icarus II embarks on a renewed attempt. The team includes physicist Dr. Robert Capa, who is responsible for the pivotal calculations for deploying and detonating the payload; Captain Kaneda (Hiroyuki Sanada), the resolute mission head; Mace, an engineer who brings a practical military viewpoint; Dr. Searle, the ship's counselor and medical officer; Cassie, the crew's empathetic pilot; tech-savvy navigator Trey (Benedict Wong); Harvey (Troy Garity), the communication officer; and botanist Corazón (Michelle Yeoh), who sustains the ship's oxygen garden, critical for life support during their arduous journey.
After 16 months in space, the mission encounters severe problems as solar noise, referred to as "the dead zone," disrupts communication with Earth. Increasingly tense, the crew is left with merely 24 hours to send final messages to their loved ones. Capa struggles with his words, unintentionally delaying Mace's efforts to craft his own message. This frustration leads to a physical confrontation between the two, creating an undercurrent of animosity that persists even after Mace apologizes. To ease tensions, Searle allows Mace to experience the simulations of Earth, fostering a temporary reconciliation as he takes time to reconnect with their home.
During crew meetings, Corazón identifies that the oxygen production from their garden has exceeded expectations, with enough stored for a successful delivery and a portion of their return journey. However, Dr. Searle becomes increasingly captivated by the sun's brilliance, defying warnings about light exposure and subjecting himself to dangerous levels of radiation.
Meanwhile, Captain Kaneda researches the logs from the Icarus I, trying to decipher the original mission's pitfalls. A distress signal from the Icarus I is picked up as they approach Mercury, prompting a debate about potentially altering their course to investigate. Mace staunchly opposes this diversion, asserting that their primary mission is to deliver the stellar payload, which outweighs any other risks.
As discussions escalate, Kaneda decides to let Capa weigh in, leaving him in a moral quandary. He validates Kaneda's plan, proposing, "Heads: two last best chances are better than one," leading to the ultimate agreement to rendezvous with the Icarus I. However, an error in calculations regarding the heat shield, missed by Trey, triggers an emergency that necessitates a spacewalk for repairs. Both Capa and Kaneda undertake this daunting task, but unforeseen challenges arise aboard the ship when Cassie inadvertently compromises their communication systems during her maneuvers.
As their oxygen garden ignites due to a cascading series of events, Mace mandates the release of the stored oxygen to extinguish the rapidly escalating fire, signifying drastic sacrifice for the crew's survival. Chaos ensues when the AI undertakes control, placing Capa and Kaneda's lives at risk as they combat the impending disaster.
Tragedy strikes when Kaneda ultimately sacrifices himself for the mission, and the crew, deprived of their oxygen source, faces harsh realities. With the dire oxygen situation worsening, the remaining crew members must make harrowing choices. Determined to save his team, Capa confronts the unexpected remnants of the Icarus I when they reach it, uncovering a haunting fate that led the earlier crew to madness.
As the harrowing ordeal unfolds, Capa discovers the treacherous Pinbacker (Mark Strong) alive aboard the abandoned ship. What ensues is a heart-stopping confrontation, testing Capa to his limits as he strives to bring the mission to fruition and confront his own doom. In a heart-wrenching climax, navigating through insurmountable odds, Capa ignites the payload, consigning himself to the sun while exuding a calm acceptance.
Back on Earth, Capa's sister watches a final message from him, while a hopeful sun breaks through the clouds, a testament to their brave sacrifice and the success of their grim mission.
