The series is set 97 years after a devastating nuclear war wiped out almost all life on Earth. The only known survivors are the residents of twelve space stations in Earth's orbit prior to the war. The space stations banded together to form a single massive station named "The Ark", where about 2,400 people live. Resources are scarce and all crimes no matter their nature or severity are punishable by death ("floating") unless the perpetrator is under 18 years of age. After the Ark's life-support systems are found to be critically failing, 100 juvenile prisoners are declared "expendable" and sent to the surface in a last ditch attempt to determine if Earth is habitable again in a program called "The 100", for which the show is named. The teens arrive on a beautiful planet they have only seen from space. Confronting the dangers of this rugged new world, they struggle to form a tentative community. However, they discover not all humanity was wiped out; some people on Earth survived the war, called Grounders, and there are even more dangers lurking – Reapers and Mountain Men.
The Ark consists of the space stations previously representing the nations of the UK, USA, Australia, Canada, China, Japan, India, Russia, Venezuela, France, Brazil, and Uganda.
In the second season, people from Mt. Weather aid several survivors after the battle at the end of the first season. Mt. Weather is filled with residents who are technologically advanced, but unable to leave their bunker due to radiation fears. While appearing outwardly extremely friendly and welcoming, they harbor dark secrets.
After crashing on Earth, survivors from the ark try to establish a new home, all the while searching for their lost children (the 100) and dealing with threats from grounders. Jaha, the remaining survivor on the ark, finds a way to leave, but ends up in an unexpected place – the ruins of New York.
Eliza Taylor as Clarke Griffin, is the daughter of Dr. Abigail Griffin and Jake Griffin. She was imprisoned as an accessory to the crimes of her father, though before, she was seen as a member of the upper class. Down on the ground, she is portrayed as relatively benevolent, possessing natural leadership qualities.
Paige Turco as Councillor Dr. Abigail "Abby" Griffin, is Clarke's mother. She is the chief medical officer of the Ark and the main proponent regarding the habitable status of Earth. Her husband was the chief engineer, Jake Griffin.
Bob Morley as Bellamy Blake, is Octavia's brother. He professes a strong determination to protect his sister, spanning from her birth. He is the initial leader of the 100, although often loses focus and control over the group. He develops from an antagonistic character to an antihero, realizing the 100 matter to him and showing compassion towards enemies and fallen comrades.
Marie Avgeropoulos as Octavia Blake, is Bellamy's sister – a rare relationship given the Ark's one-child rule. She was kept a secret by her family, living under the floor to avoid detection by authorities. She is a fiercely independent girl who is constantly rebelling and getting attention from men, most notably the grounder Lincoln.
Thomas McDonell as Finn Collins, is a caring teenager who is always looking for fun and a peaceful solution. Originally Clarke's love interest, the relationship starts collapsing when he is later revealed to be Raven's boyfriend. He begins the series more interested in helping others as than seeking vengeance, however in season two, fear of losing the remaining 100 cause him to question his ideals and he becomes more unpredictable and violent.
Lindsey Morgan as Raven Reyes (recurring, season 1; regular, season 2), is a zero-gravity mechanic when on board the Ark. She later was launched in a pod sent to Earth, despite not being a teenage criminal. Brave, intelligent, and self-secure, Raven doesn't back away from a challenge, no matter the cost.
Devon Bostick as Jasper Jordan, is a goofy, outgoing and adventurous chemist who would risk his life to save the other 100. After his capture and subsequent rescue from grounders, he is anxious in addition to his other personality traits, and is the most susceptible to comforts where others – like Clarke – would grow wary.
Ricky Whittle as Lincoln (recurring, season 1; regular, season 2), a grounder who initially belongs with his tribe, but later rescues Octavia and they fall in love. He betrays his own tribe so Octavia can live, but he leaves when he knows a war is coming, though eventually returning for her.
Christopher Larkin as Monty Green, is a smart and tech-savvy teenager who is a valuable asset to the teenagers on Earth due to his intellect. His family is in charge of the pharmaceuticals on the Ark. He is best friends with Jasper, remaining loyal through thick and thin.
Henry Ian Cusick as Councillor Marcus Kane, is the Chancellor's second-in-command. Though he initially supports population culls to extend life on the Ark, he later has a change of heart and will not rest until everyone left alive is safe.
Isaiah Washington as Chancellor Thelonious Jaha, is Wells' father and the elected leader of the Ark.
Eli Goree as Wells Jaha (season 1; guest, season 2), is Clarke's former best friend and son of the Ark's chancellor – a relationship for which he is despised by the rest of the 100. He intentionally committed a crime when he found out about the imminent launch to join the expedition to Earth and try to protect Clarke, for whom he harbors feelings. Clarke believed he was the reason her father is dead.
Richard Harmon as John Murphy (season 1–), is Bellamy's main thug and generally disliked by the group. After being wrongfully accused and nearly lynched, he was banished from the camp. He later returns, having escaped from the grounders, although he has been beaten severely. He seems to have come around and forgiven the 100 for banishing him, but later he kills two of their members in revenge and shoots another, proving his unpredictability. In season 2 he proves his worth to the 100, especially Bellamy and Finn
Keenan Tracey as Sterling (season 1–2), is one of the 100 who fell to his death.
Jarod Joseph as Nathan Miller (season 1–), is one of the 100. He is among the 47 held in Mount Weather.
Chelsey Reist as Harper (season 1-), is one of the 100. She is among the 47 held in Mount Weather.
Rhys Ward as Atom (season 1), is one of the 100 who is a close adviser to Bellamy and later became romantically involved with Octavia. He was severely burned by the an acid fog spell on Earth and had to be killed out of mercy.
Brendan Meyer as Myles (season 1–2), is one of the 100, who was asphyxiated by John Murphy.
Josh Ssettuba as Connor (season 1), is one of the 100, who was also killed by John Murphy.
Genevieve Buechner as Fox (season 1), is one of the 100.
The Ark and Camp Jaha
Sachin Sahel as Doctor Jackson (season 1-), is Abby's helper and confidant.
Chris Browning as Jake Griffin (season 1), is Abby's husband and Clarke's father. He was executed for attempting to release critical information regarding the Ark's swiftly deteriorating life-support systems.
Terry Chen as Commander Shumway (season 1), is loyal to Councilwoman Diana. His plan to have Bellamy assassinate Jaha is later revealed, and Shumway is killed on Diana's orders.
Kate Vernon as Councilwoman Diana Sydney (season 1), is the ex-chancellor of the Ark. She and her team steal and launch the Exodus ship to Earth. The ship crash lands, however, and she is presumed dead.
Kendall Cross, as Major Byrne (season 2), is captain of Marcus Kane's guard at Camp Jaha.
The series premiere was watched by an estimated 2.7 million American viewers, and received an 18–49 rating of 0.9. It is considered the most-watched show in its time-slot on The CW since 2010 with the series Life Unexpected. On Rotten Tomatoes, the show was certified "fresh", with 72% of professional reviewers reviewing it positively, with a consensus of "Although flooded with stereotypes, the suspenseful atmosphere helps make The 100 a rare high-concept guilty pleasure." On metacritic, it scores 63 out of 100 points.
Brian Lowry of The Boston Globe said, "Our attraction to Apocalypse TV runs deep, as our culture plays out different futuristic possibilities. That's still no reason to clone material, nor is it a reason to deliver characters who are little more than stereotypes." Allison Keene of The Hollywood Reporter gave a negative review to the show, and writes "The sci-fi drama presents The CW's ultimate vision for humanity: an Earth populated only by attractive teenagers, whose parents are left out in space." Kelly West of Cinema Blend gave it a more positive review while noting, "CW's Thrilling New Sci-fi Drama Is A Keeper. CW's The 100 seeks to explore that concept and more with a series that's about equal parts young adult drama, sci-fi adventure and thriller. It takes a little while for the series to warm up, but when The 100 begins to hit its stride, a unique and compelling drama begins to emerge."IGN also gave the show a more positive review. The article, written by Eric Goldman, gave the initial episodes lower scores, but the fifth episode four out of five stars.
Young Actor in a TV Series Drama or Comedy Guest Starring or Principal Role
The first season of The 100 was licensed exclusively to Netflix in Canada. In New Zealand, the series premiered on TVNZ's on-demand video streaming service on March 21, 2014. In South Africa, it premiered on M-Net Series Showcase HD on June 4, 2014. In the UK and Republic of Ireland, The 100 premiered on E4 on July 7, 2014. The first episode was viewed by an average audience of 1.39 million, making it the channel's biggest ever program launch. The show also airs in Singapore, on MediaCorp Channel 5, 19 days behind the original telecast. In Australia, The 100 was originally scheduled to premiere on Go!, but premiered on Fox8 on September 4, 2014. In Brazil, the show was set to launch on MTV in October 2014. In Denmark, it will air on Kanal 5, but is yet to premiere.