On the other side of every mirror on Earth lies the parallel dimension of Ventara – where its civilians were abducted by Xaviax from the planet Karsh. Eubulon, who once helped his General, decided to counter with the Kamen Riders, who fought Xaviax with armor and abilities powered by their Advent Decks. Unfortunately, Xaviax defeated all the Riders except for two when they were sleeping, thanks to tricking the original Dragon Knight. He took the Advent decks and targeted Earth to get more humans to enslave to rebuild his home planet. Len, Kamen Rider Wing Knight is the only man who can stop Xaviax from destroying Earth. To defeat Wing Knight, Xaviax gave the Advent decks to the doppelgängers of the Riders, as the decks were locked with their DNA, and tricked them to carry out his evil plans by exploiting their vices. To vent is to send a Rider into the Advent Void, in which they can never escape, sent through a final vent or a super-powered attack.
After his mysterious disappearance of his father, Kit Taylor finds an Advent Deck in his father's apartment and becomes the second Kamen Rider Dragon Knight. He teams up with Len to vent the corrupt Kamen Riders and sends Xaviax's monsters. The two hope to not only save the Earth from Xaviax's domination, but to find Kit's missing father and win back Ventara as well. Maya Young, a young reporter from a blog, is sucked into Ventara and is able to see the monsters. Maya assists Len and Kit, eventually discovering that Kit's dad was drained of his energy, most likely by Xaviax and supposedly there was no cure. Kit is tricked by con-man Drew, Torque, into believing Len is the true traitor. Xaviax makes the mistake of also recruiting Grant, Kamen Rider Camo, into venting Len, which he never gets to do as he fights Drew. Drew in turn vents Grant and Xaviax blackmails racer Brad Barrett into joining what he thinks is a Battle Club.
When Brad finds out the truth, he tries to quit but is threatened into venting Len. Len tells Maya the truth about Drew and she leaves a message to Kit. Meanwhile, legendary Blogger JTC, that Maya and Trent look up to, gives Maya pictures of Kamen Rider Sting, who is protecting Gramercy Heights. Sting attempts to arrest Kit, believing he is an invading alien. Sting, Chris Ramirez, is unable to stay a marine because of his extreme asthma. Len has a chance to vent Chris but doesn't. Xaviax confronts Drew on his betrayal and sends the mysterious Strike after him but Drew manages to elude him. Strike vents Brad because he is unable to vent Len and later vents Drew. Chris joins forces with Len & Kit. Believing that JTC knows too much, Len wants to meet him. Sting fights with Strike and is rescued by Kit and Len.
Chris considers quitting but reconsiders after a lesson in teamwork by Len. JTC seems to be kidnapped by monsters and Strike attacks Len, Kit, and Chris. The Cho Brothers, recruited to be Spear and Axe by Xaviax, attack Len who defeats them easily. Kit goes back to the hospital, finds that his father is missing and that Strike took him. JTC confronts Chris and offers him a cure for his asthma if he betrays Len. Chris refuses and they fight for a third time, nearly gets vented but manages to escape. Kit arrives at the bookstore and tells Maya, Trent & Lacey about his father's abduction when the Cho Brothers show up looking for a fight. Kit, Len, Chris and Danny enter Ventara while Albert transforms in the bookstore proving to Trent and Lacey that the Kamen Riders exist.
This makes Lacey scared and when Michelle Walsh, who got Maya fired from the blog for supposedly fraudulent reporting, asked her to take Mayas info from her laptop, Lacey agreed. Kit takes on Axe while Chris and Len take on Spear, his Advent Beasts and mirror monsters. Kit has a chance to vent Axe but doesn't. Strike arrives and tries to vent Len but Chris takes the hit and is vented instead, which makes Len & Kit feel guilty and upset. At home, Kit goes through Chris' bag, finds and reads a letter he wrote to his father. Kit fights the Cho Brothers who tease about Chris, but Kit is easily overpowered. Kit returns to the bookstore as JTC calls with the location of his father. Maya decides to go with Kit to help him get his dad back. Kit and Maya find Kit's dad with JTC who makes Kit an offer: his dad for Wing Knight's Advent Deck.
When Kit refuses, the Cho Brothers show up and Maya manages to escape with Kit's dad. Kit manages to fight off Strike and Axe before he vents Spear. Lacey delivers Maya's computer information to Michelle while Kit goes to the hospital to see his father. Danny vows to get Kit for venting his brother and Xaviax threatens Danny to make sure that he doesn't. Kit is attacked by Axe until Kamen Rider Siren intervenes, but Danny escapes when he is overpowered. She then attacks Kit, thinking he also works for Xaviax. Len arrives to protect him and is surprised to discover she is his girlfriend Kase. Kase tells Len about how she survived before being introduced to Kit. Kase tests him to see if he has what it takes to be a real Kamen Rider, and Kit loses. JTC confronts Danny about disobeying Xaviax and nearly vents him before deciding to give him one last chance.
At home, Kit continues to feel frustrated and is then attacked by Kamen Rider Onyx, finding out it is dream. Michelle introduces herself to Kit, calling him Dragon Knight, revealing herself to be a government agent. At the hospital, Kit meets Xaviax who briefly wakes his father and uses him as an incentive for Kit to vent Wing Knight, which Kit seems to consider. He reveals he has been sending him the messages as his father and is the one has caused his police trouble because he had no known vices. In a dream, Kit vents Len & Kase as Onyx. Trent tells Maya to delete her computer information which pains her so. Danny attacks Kit and Strike vents Danny. Lacey reveals to Maya and Trent she stole Maya's computer information. They feel betrayed by Lacey but forgive her before Maya realizes she must warn Kit, disguised as Lacey as the No-Men are spying on them. Xaviax possesses the body of Vic Fraser so he can use his Wrath armor and trashes the bookstore along with Strike in a battle against Len & Kase. Kit is kidnapped, taken to a secret government facility and thrown in a cell.
Trent calls two of his friends to help hack the No-Men's Database and find out where Kit is being held, Maya goes to find him. After Len and Kase arrive, Kit manages to escape using a cup of water he spills on the floor while being interrogated by Michelle. Len, Kase and Maya escape the No-Men themselves. Wrath and JTC then attack Kit, injuring his leg, before Len and Kase arrive to save him. At night they all meet in the bookstore, Kase heals Kit injured leg, Michelle and the agents show up forcing them to escape through a mirror into Ventara. Kase gives Len and Kit their Survive Mode cards. JTC steals the Thrust and Sting decks from Len before escaping. After the Riders follow him, Michelle kidnaps Maya, Trent and Lacey. Kit fights Strike while Len and Kase fight Wrath. When Strike gains the upperhand on Kit, nearly venting him, Len shows up and saves him. Strike reveals his combo of three Advent Beasts.
Kase arrives in time to help but is vented by Strike. Kit finds Strike and fights him until Len arrives and uses his Survive Mode card. Len eventually vents JTC. Kit and Len come up with a plan to vent Xaviax but Kit's Survive Mode wears off. He tries using his Final Vent but misses, is hit with Wrath's Final Vent and is vented. Wrath is also vented but Xaviax survives, claims Kit's deck and escapes. Michelle arrives with Trent, Maya and Lacey, they hug Len at the loss of Len. Michelle takes them to meet the Advent Master, Eubulon in the No-Men's Headquarters for Len to awaken him. Meanwhile, Xaviax recruits Adam from the illusion of Ventara he is in with Sarah, his girlfriend, to once again become Dragon Knight. While Trent becomes a No-Men to help locate Xaviax's transmitters, Eubulon makes Maya the new Kamen Rider Siren.
After Len destroys a transmitter, he is confronted by Adam. Eubulon manages to rescue Ventaran Riders Torque, Strike and Axe from the Advent void. Len and Eubulon explain to them what happened to Ventara and what is happening to Earth. The Kamen Rider's mission is to deactivate Xaviax's transmitters so he cant kidnap Earthlings. Eubulon manages to rescue Kit and Kase from the void, he gives Kit the Onyx deck, which he is not sure of at first but later learns to accept. Kit nearly vents Adam but is stopped by Eubulon, who decides to give Adam a chance to explain. Len refuses to forgive Adam but Eubulon calms him down. Maya returns the Siren Deck to Kase and Adam tells how he was tricked by Xaviax. Xaviax disguises himself as Eubulon, berates and fights him. Xaviax orders Adam to let Len disable the transmitter to give the others a false sense of security and then vent him. Eubulon continues to fix the Advent Key.
After Len disarms the transmitter, Adam attacks him and they fight. Adam tells Len that there's no beating Xaviax, and that he won't give up on Sarah for anything. Adam prepares to vent Len but can't bring himself to do it. Just then, Kit and Kase arrive at the scene, Adam tells them to vent him, but they refuse. Adam confesses about the transmitters. Eubulon lets Adam fight with the others. Maya and Trent sneak into Xaviax's lair to create a mirror portal for the Riders, because there is a forcefield that doesn't let Riders inside. The Kamen Riders rush into Xaviax control room and are joined by the Vantaran Riders brought back from the Advent Void. Finally all 13 Ventaran Kamen Riders unite their final vents in order to finally destroy Xaviax. Maya recounts everything that has happened in her new book as a fictional children's novel. All the Ventarans are brought back to their homes, the Earth Kamen Riders are returned from the Advent Void, Adam told the truth to Sarah and she forgave him, and Kit becomes Kamen Rider Dragon Knight for good.