A British Special Boat Service (SBS) was created in WWII by the British Navy. It was very effective in neutralizing clandestine enemy positions and was often assigned to work alongside the MI6. It became the basis for the US Military Navy Seals. John Stratton (Dominic Cooper), is an SBS soldier, working for the Intelligence Detachment in Northern Ireland. Aggy (Gemma Chan), Spinks (Jake Fairbrother) and Cummings (Tom Felton) are mission specialists who are assisting John in infiltrating an Iranian bio-chem weapons factory and to neutralize it. But when john & his partner Marty (Tyler Hoechlin) get there, they find all the workers of the factory shot dead and the bio-chem weapons vault empty. They are late and compromised. They come under attack even as their evacuation is under way and John's partner is shot in the neck during the escape. He dies.Now John lives in Ireland on his boat with Ross (Derek Jacobi) his neighbor and the only friend in the world. Sumner (Connie Nielsen), John's boss in MI6 calls him in and tells him that an unknown amount of WMD stolen from the facility is now in play and she needs him to track down the terrorist behind it. John meets Aggy to look at the video footage of their rescue to figure out who the main man behind their attack was. Hank (Austin Stowell) is assigned to John as his new partner.Sumner identifies the main attacker as Grigory Barovsky (Thomas Kretschmann) who is ex-FSB and was supposed to be dead for 20 yrs. Sumner reveals that 20 years ago Barovsky was a rising FSB agent, so MI6 painted him as a double agent and got the FSB to presumably assassinate their best man. But looks like that wasn't so. Barovsky tests the bio-chem weapons on a community in Ukraine, which kills 150 people in an instant. The weapon is designed to be deployed aerially, where it can cause 100 times the damage with a tiny amount at a height of 1000 ft. Only Tariq Alawi (Igan Naor) is a bomb maker, specialized enough to turn raw bio-chem into a bomb version that can be deployed from a plane.The team abducts Alawi, who admits that he already weapon activated the bio-chem now called Satan's snow. Alwai is forced to reveal the next link to Barovsky, by the name of Greco, an Italian engineer (Rinat Khismatouline). They find that Greco has engineered the design provided by Alawi and created a areal dispersion drone prototype for Satan's snow. When Barovsky comes to pick up the drone, he finds the cameras installed by Hank & John & thinks Greco is double crossing him. So he kills Greco & tries to leave with the drone. Hank disobeys orders from John and attacks Barovsky. They get the drone, but Barovsky escapes.John theorizes that Barovsky already has 3 other drones already delivered to him. John meets Sumner & discusses how Sergei (Olegar Fedoro) was Barovsky's boss when FSB gave the order to shoot him. John tells Sumner that Sergei perhaps didn't shoot Barovsky and instead used him now (when Sergei is the Minister of Interior) to compromise Britains Iram mission that would have put 2 dead Brits inside Iranian space and led to international scandal from which Moscow would profited handsomely. But Barovsky went rouge, kills Sergei, and keeps the bio-chem for his own profit rather than handing it over to Moscow. John also asks Sumner about her past relationship with Barovksy. Sumner admits that she fell in love with him when she was assigned to the mission of proving Barovsky as a double agent.Turns out while killing Sergei, Barovksy stole the file on all his foreign agents, which included Cummings. Barovsky now contact Cummings to steal the 4th drone and deliver it back to him, else he would expose Cummings as a Russian agent to Sumner. Sumner suspects that Barovsky could decide to attack Moscow with Satan's Snow to extract his revenge. Cummings pitches to Sumner to share this intel with the FSB in exchange for an US hostage in Russia. Sumner agrees, but when John & Hank head there, they find that the exchange is not the US hostage (didn't have a tattoo at the right place). They shoot the FSB imposters & return. They finally deduct that Cummings is the infiltrator in their group. They are confused as to why Barovsky would go to such lengths for a drone that he doesn't even need.Meanwhile Cummings goes missing. Spinks finally cracks the guidance system on the 4th drone and figures out that the target is not Moscow, but London. Sumner tracks & finds the ship through which Barovksy is entering London with the other 3 drones. She orders John to intercept. They do, & recover 2 drones, but Barovsky still escapes with the last one. Meanwhile Cummings abducts Aggy, & takes her to a warehouse & shoots himself. Shortly thereafter Barovsky enters the warehouse with the last drone. The warehouse has a London bus inside, which Aggy uses to hide. he loads the drone on the bus. Aggy helps John & Hank track the bus down in downtown London. They manage to stop the bus, kill Barovksy and neutralize the agent before Barovksy could release it. Later, John asks Aggy out for a date.